Let the channel "caught up" mark replace ordinary message read marks - #590

🤖

## Summary
The web and desktop app (`buzz-app`) saves a "caught up" mark when you
scroll to the bottom of a channel and stay there, and another one for a
thread. Mobile did not understand these marks. It relied on the separate
per-message read marks that `buzz-app` also kept. `buzz-app` is about to
stop keeping those per-message marks under the "caught up" mark
(block/buzz-app#590), to save space in the synced read state.

With this change, mobile reads the "caught up" marks with the same rules
as `buzz-app`:
- `activity:<channel>` reads ordinary top-level messages up to its time.
It does not read thread replies, mentions, broadcasts or DM messages,
because reaching the bottom of the channel does not prove you saw those.
- `thread-activity:<root>` reads the replies in that thread up to its
time.

So a message you read in `buzz-app` stays read on mobile: in the app
badge, the channel list unread state, the "new messages" divider when
you open a channel, and the Mark read / Mark unread action.

Mobile does not write these marks. It only reads them.

### Related issue
None found. Companion to block/buzz-app#590. The old desktop app in this
repo is intentionally not changed.

### Testing
- New badge tests: `activity:` clears an ordinary top-level message, and
does not clear a mention, a thread reply or a DM. `thread-activity:`
clears replies in its own thread only. The two "clears" tests fail
without this change.
- New tests for the per-message read check and the "ordinary message"
rule.
- `flutter analyze`, `dart format` check and the full mobile test suite
pass locally.
- Not yet exercised on a device.

---------

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

🤖

## Summary
The web and desktop app (`buzz-app`) saves a "caught up" mark when you
scroll to the bottom of a channel and stay there, and another one for a
thread. Mobile did not understand these marks. It relied on the separate
per-message read marks that `buzz-app` also kept. `buzz-app` is about to
stop keeping those per-message marks under the "caught up" mark
(block/buzz-app#590), to save space in the synced read state.

With this change, mobile reads the "caught up" marks with the same rules
as `buzz-app`:
- `activity:<channel>` reads ordinary top-level messages up to its time.
It does not read thread replies, mentions, broadcasts or DM messages,
because reaching the bottom of the channel does not prove you saw those.
- `thread-activity:<root>` reads the replies in that thread up to its
time.

So a message you read in `buzz-app` stays read on mobile: in the app
badge, the channel list unread state, the "new messages" divider when
you open a channel, and the Mark read / Mark unread action.

Mobile does not write these marks. It only reads them.

### Related issue
None found. Companion to block/buzz-app#590. The old desktop app in this
repo is intentionally not changed.

### Testing
- New badge tests: `activity:` clears an ordinary top-level message, and
does not clear a mention, a thread reply or a DM. `thread-activity:`
clears replies in its own thread only. The two "clears" tests fail
without this change.
- New tests for the per-message read check and the "ordinary message"
rule.
- `flutter analyze`, `dart format` check and the full mobile test suite
pass locally.
- Not yet exercised on a device.

---------

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

…iles

* origin/main:
  Install the bundled Buzz CLI skill on desktop startup (#667)
  refactor: share status, DM and sidebar intent policy across hosts (#683)
  fix(sessions): show enabled page in navigation (#648)
  fix(composer): replace exact emoji shortcodes on closing colon (#584)
  refactor(mentions): let the Mentions plugin own what the chooser offers (#669)
  Refresh agent cards and move saved teams to Agents (#656)
  Let the channel "caught up" mark replace ordinary message read marks (#590)
  test(rust): create test executables without an open write descriptor (#665)
  feat: add mobile pairing Settings plugin (#595)
  Cache the floating action bar's MediaQueryList (#652)
  feat(messages): copy and paste mentions, channel refs and links (#579)
  Remember desktop window size and position across launches (#655)
  fix(messages): show verified workflow ownership separately from signer (#663)
  fix(activity): collapse duplicate channel-wide agent indicators (#664)
  docs: make native development the acceptance path (#666)
  Sign plugin releases with NIP-PS (#465)
  test(browser): explain WebKit fetch refusals during a page reload (#650)
  Add Claude Code harness setup in Settings (#654)
  fix: restore workflow controls, run feedback and workflow message authors (#661)

Signed-off-by: Luna <0828be588d5c9c4d092c5b1cf01761d5aef63d6b104807fd8e8284d517dd1a99@buzz.block.builderlab.xyz>