feat(desktop): quote selected timeline text into the composer (beta) - #56068

Brings timeline quoting from #55664 onto beta so it ships in the beta desktop build. Same four commits, cherry-picked cleanly onto current beta with no conflicts or changes.

• Select text anywhere in the timeline (messages, tool output, diffs, cmd+a) and click Quote to add it to the composer as a chip; plain text only, capped at 20k chars.
• ⌥-click Quote adds the quote and opens a focused comment box; hovering a chip shows Add comment to comment on just that quote.
• Quotes are stored in message metadata, so the sent message shows quote cards and revert/fork restores them to the composer.
• The model receives each quote as > lines followed by its comment.
• Quoting is disabled in shell mode.

#55664 stays open against dev.

Checks run on this branch: tsgo typecheck for packages/app, packages/session-ui and packages/ui, and packages/app unit tests (761 pass). The e2e spec added in timeline-stream.spec.ts was not run locally.

Requested by: Filip via Slack