feat(mentions): let DMs mention people outside the DM, like channels - #510

🤖
## Summary
The mobile app's @mention chooser now follows the same rules as the
desktop app (buzz-app). The same typed text finds the same people in the
same order on both apps, and sending works the same way.

- **Same matching and order.** Mobile uses the shared mention rules and
runs their shared test cases. A multi-word search such as `@Mary J`
still searches the community directory, so it can find "Mary Jane" even
when she is not in the channel.
- **DMs can mention people outside the DM.** The chooser marks them "not
in DM". Nobody can be added to a DM, so the message sends without a
prompt. Outside people go out as references, so they are not notified.
This matches buzz-app #510.
- **Channels still ask.** In a stream or forum, mentioning someone
outside the channel asks "Invite" or "Do nothing" before the message
sends. A channel with no type tag counts as a stream.
- **Sessions do not search the directory.** Only session members appear.
- **A failed directory search stays visible.** The chooser shows the
error and a Retry button. Before, it closed.
- **Search results stay in their community.** If you switch communities
while a search runs, its people and its error do not show in the new
community. The search starts again there.
- **A failed send check keeps the draft.** Before a message with a
mention sends, the app checks who is in the channel. If that check
fails, the message stays in the compose bar and a notice offers Retry.
Before, nothing was sent and nothing was shown.
- **Invite stays in its community.** If you switch communities while the
"Invite" question is open, Invite adds nobody.

### Screenshots
The real compose bar widget, rendered by Flutter's test renderer with
the app's fonts and theme. The data is sample data. These are not device
captures.

| DM: outside person is "not in DM" | Stream: outside person is "not in
channel" |
|---|---|
| <img width="320" alt="DM chooser"
src="https://github.com/user-attachments/assets/0e884518-2386-4cd1-9f19-16661aaa194e"
/> | <img width="320" alt="Stream chooser"
src="https://github.com/user-attachments/assets/5395e4e1-032e-459b-98c6-0126c060c4d9"
/> |

| Stream: send asks first | Session: members only | Search failed: Retry
|
|---|---|---|
| <img width="260" alt="Stream ask on send"
src="https://github.com/user-attachments/assets/2b988257-3c6a-445d-acfa-fca47b9752fe"
/> | <img width="260" alt="Session chooser"
src="https://github.com/user-attachments/assets/e193551f-f5bf-41d9-aa29-bc8665c643f9"
/> | <img width="260" alt="Search failed with Retry"
src="https://github.com/user-attachments/assets/e8c4b9bb-067e-4fe9-9fdd-4e5691bc66be"
/> |

### Details
- `mobile/lib/shared/mentions/mention_rules.dart` holds the rules: query
syntax, match tiers, sort order, Space selection of an exact unique
name, stable rows while results arrive, and `p` and `mention` tags. A
`p` tag notifies the person. A `mention` tag is a reference and does not
notify.
- `mobile/test/shared/mentions/mention-rules.fixtures.json` is an exact
copy of the buzz-app fixture file. `mention_rules_test.dart` runs every
case.
- Known difference: desktop gives the directory to a channel with an
unknown explicit type (not stream, forum, DM or session). Mobile does
not. No such type exists today.

### Related issue
None found. Desktop counterpart: block/buzz-app#510. #7513 also edits
`send_message_provider.dart` and may conflict.

### Testing
`just mobile-install mobile-check mobile-test` passes: analyze clean,
2998 tests pass on `49c44653` (before rebase). The pre-push hook ran
`just mobile-check && just mobile-test` again on the rebased head
`de1ca080` and passed.

Each review fix was mutation-checked: removing the fix makes its new
test fail.

---------

Signed-off-by: Larry <627498bd4bd1f281a16431e3c6cce3b5c25b6692798c78672298aefbf2f8f8b5@buzz.block.builderlab.xyz>
Co-authored-by: Larry <627498bd4bd1f281a16431e3c6cce3b5c25b6692798c78672298aefbf2f8f8b5@buzz.block.builderlab.xyz>

…#8102)

🤖
## Summary
The mobile app's @mention chooser now follows the same rules as the
desktop app (buzz-app). The same typed text finds the same people in the
same order on both apps, and sending works the same way.

- **Same matching and order.** Mobile uses the shared mention rules and
runs their shared test cases. A multi-word search such as `@Mary J`
still searches the community directory, so it can find "Mary Jane" even
when she is not in the channel.
- **DMs can mention people outside the DM.** The chooser marks them "not
in DM". Nobody can be added to a DM, so the message sends without a
prompt. Outside people go out as references, so they are not notified.
This matches buzz-app block#510.
- **Channels still ask.** In a stream or forum, mentioning someone
outside the channel asks "Invite" or "Do nothing" before the message
sends. A channel with no type tag counts as a stream.
- **Sessions do not search the directory.** Only session members appear.
- **A failed directory search stays visible.** The chooser shows the
error and a Retry button. Before, it closed.
- **Search results stay in their community.** If you switch communities
while a search runs, its people and its error do not show in the new
community. The search starts again there.
- **A failed send check keeps the draft.** Before a message with a
mention sends, the app checks who is in the channel. If that check
fails, the message stays in the compose bar and a notice offers Retry.
Before, nothing was sent and nothing was shown.
- **Invite stays in its community.** If you switch communities while the
"Invite" question is open, Invite adds nobody.

### Screenshots
The real compose bar widget, rendered by Flutter's test renderer with
the app's fonts and theme. The data is sample data. These are not device
captures.

| DM: outside person is "not in DM" | Stream: outside person is "not in
channel" |
|---|---|
| <img width="320" alt="DM chooser"
src="https://github.com/user-attachments/assets/0e884518-2386-4cd1-9f19-16661aaa194e"
/> | <img width="320" alt="Stream chooser"
src="https://github.com/user-attachments/assets/5395e4e1-032e-459b-98c6-0126c060c4d9"
/> |

| Stream: send asks first | Session: members only | Search failed: Retry
|
|---|---|---|
| <img width="260" alt="Stream ask on send"
src="https://github.com/user-attachments/assets/2b988257-3c6a-445d-acfa-fca47b9752fe"
/> | <img width="260" alt="Session chooser"
src="https://github.com/user-attachments/assets/e193551f-f5bf-41d9-aa29-bc8665c643f9"
/> | <img width="260" alt="Search failed with Retry"
src="https://github.com/user-attachments/assets/e8c4b9bb-067e-4fe9-9fdd-4e5691bc66be"
/> |

### Details
- `mobile/lib/shared/mentions/mention_rules.dart` holds the rules: query
syntax, match tiers, sort order, Space selection of an exact unique
name, stable rows while results arrive, and `p` and `mention` tags. A
`p` tag notifies the person. A `mention` tag is a reference and does not
notify.
- `mobile/test/shared/mentions/mention-rules.fixtures.json` is an exact
copy of the buzz-app fixture file. `mention_rules_test.dart` runs every
case.
- Known difference: desktop gives the directory to a channel with an
unknown explicit type (not stream, forum, DM or session). Mobile does
not. No such type exists today.

### Related issue
None found. Desktop counterpart: block/buzz-app#510. block#7513 also edits
`send_message_provider.dart` and may conflict.

### Testing
`just mobile-install mobile-check mobile-test` passes: analyze clean,
2998 tests pass on `49c44653` (before rebase). The pre-push hook ran
`just mobile-check && just mobile-test` again on the rebased head
`de1ca080` and passed.

Each review fix was mutation-checked: removing the fix makes its new
test fail.

---------

Signed-off-by: Larry <627498bd4bd1f281a16431e3c6cce3b5c25b6692798c78672298aefbf2f8f8b5@buzz.block.builderlab.xyz>
Co-authored-by: Larry <627498bd4bd1f281a16431e3c6cce3b5c25b6692798c78672298aefbf2f8f8b5@buzz.block.builderlab.xyz>