feat: broken-anchor indicator for orphaned annotations

[leotrs]

Summary

• When an annotation's anchor can't be resolved (text deleted, structure changed), the card now shows a visual "Original text no longer found" warning banner with an AlertTriangle icon
• Orphaned cards get a dashed left border and reduced opacity to distinguish them from anchored annotations
• DockableAnnotations tracks orphaned IDs during sort and passes orphaned prop to Note component

Test plan

• Verify orphaned annotations show the warning banner in the sidebar
• Verify anchored annotations do NOT show the banner
• Verify delete still works on orphaned annotations
• Verify collapsing an orphaned card hides the banner (shows collapsed preview)
• 12 new unit tests covering orphan detection and UI rendering

🤖 Generated with Claude Code

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[leotrs]

Needs human review

What changed: Orphaned annotations now show a visual "Original text no longer found" warning banner with dashed border and reduced opacity. Per-message edit/delete buttons added to annotation cards. Shared card design overhauled (removed top border, added subtle shadow). Delete blocked on shared annotation messages (new 403). updated_at column added to annotation messages with "edited" indicator.

Review checklist:

1. Open the deploy preview: https://deploy-preview-352--rsm-studio-frontend.netlify.app
2. Open a document with annotations, then delete the anchored text for one annotation — verify the orphaned card shows a dashed left border, reduced opacity, and an orange "Original text no longer found" banner
3. Hover over the orphaned card — verify opacity returns to full
4. Collapse the orphaned card — verify the banner disappears (only collapsed preview shown)
5. Click delete on the orphaned card — verify it still works
6. On a private annotation, activate the card and verify Edit and Delete buttons appear below the note text
7. Edit a private note — verify the inline textarea appears, Save/Cancel work, and an "edited" tag appears after saving
8. Delete a private note — verify the two-click confirmation pattern (first click arms, second click deletes)
9. On a shared annotation thread, activate the card and verify the inline edit pencil icon appears on hover for your own messages
10. Edit a shared thread message — verify the inline textarea and "edited" tag behavior
11. Verify there is no delete button on individual shared thread messages
12. Check that the shared card redesign looks correct: no top colored border, subtle shadow instead of border, italic quoted text with background

What to look for:

• Orphan banner alignment, icon color (orange), and text readability
• Dashed border style and opacity transition on orphaned cards
• Edit/delete buttons only appear for your own messages
• "edited" tag with middle-dot separator renders correctly in both header (private) and thread message header (shared)
• Shared card shadow looks clean — no double borders or visual artifacts
• Avatar size change (16px → 12px) doesn't break layout elsewhere
• No regressions on non-orphaned annotation cards