fix(desktop): make ⌘K open the composer link editor for selections

[AaronGoldsmith]

Problem

The formatting toolbar's link button advertises ⌘K in its tooltip, and the keyboard-shortcuts settings panel lists "Insert link ⌘K" — but the binding was never implemented. The window-level quick-search handler in AppShell owned ⌘K unconditionally, so pressing it with composer text selected opened the channel search dialog instead of the add-link modal.

(A code comment even said "Media paste + ⌘K link shortcut via Tiptap editorProps" — but only the paste half was ever written.)

Fix

Bind ⌘K in the editor's raw ProseMirror handleKeyDown hook (the same proven layer as the ArrowUp edit-last-message handling) and consume it only when it applies:

State	⌘K behavior
Composer text selected	Opens Add link dialog seeded with the selection
Caret inside an existing link	Opens Edit link dialog seeded with that link
Empty caret in composer	Falls through to quick search (unchanged)
Unselected drafted text (caret only)	Falls through to quick search (unchanged)
Focus outside composer	Quick search (unchanged)
Plain Ctrl+K on macOS	Emacs kill-line (unchanged)

The conditional fall-through deliberately preserves the app-wide ⌘K quick-search muscle memory — the shortcut only claims the key when there's something to link.

Changes

• useRichTextEditor.ts — new onLinkShortcut option handled in editorProps.handleKeyDown, with event.repeat / event.isComposing guards. hasPrimaryShortcutModifier still rejects Ctrl on macOS, so the macEmacsTextShortcuts Ctrl-K kill-line binding is untouched.
• useLinkEditor.tsx — new openFromShortcut(): opens the dialog only when a selection or caret-adjacent link exists, returns whether it consumed the shortcut, and dismisses the link hover card first.
• MessageComposer.tsx + ForumComposer.tsx — wired via the existing onEditLinkRef pattern (both useRichTextEditor consumers covered).
• AppShell.tsx — global shortcut handler now bails on event.defaultPrevented (composer wins when it claims the key) and on event.repeat (held ⌘K can't stack quick-search over the link dialog).
• keyboard-shortcuts.ts — format-link description updated to match actual behavior.
• composer-link-shortcut.spec.ts — new Playwright spec covering all six states in the table above, registered in the smoke project.

Testing

• ✅ 6/6 new e2e tests pass (including the mac Ctrl+K kill-line regression guard)
• ✅ Full smoke regression sweep (smoke / spoiler / messaging / navigation): 68 passed
• ✅ Unit tests: 2079 pass · tsc clean · biome clean (6 pre-existing warnings, unchanged)
• ✅ Two independent adversarial review passes — both APPROVE WITH NITS; all nits addressed (repeat/IME guards, kill-line regression test, shortcut description accuracy, comment accuracy, handler simplification)

Mechanism notes (for reviewers)

ProseMirror attaches its keydown listener directly to the editor element; when handleKeyDown returns true, ProseMirror calls event.preventDefault() and the event still bubbles to the window with defaultPrevented === true, which the AppShell listener now respects. When the handler returns false the event bubbles untouched and quick search opens exactly as before.

[wesbillman]

Approve — reviewed on behalf of Wes (verified locally at head aba9752).

Verification: checked out the PR head and ran all 24 search-related smoke e2e tests (every existing ⌘K quick-search flow) plus the 6 new composer-link-shortcut specs — all green; tsc clean.

Why the ⌘K change is safe: the editor consumes the key only when openFromShortcut() finds a selection or caret-on-link; otherwise it returns false, no preventDefault(), and the event bubbles to AppShell's quick-search listener exactly as before. ⌘⇧K (new DM) and macOS Ctrl+K kill-line verified untouched. The selection→link / no-selection→palette discriminator matches GitHub/Notion/Linear precedent, and it's the behavior the toolbar tooltip and shortcuts panel have advertised all along.

One note: the main CI workflow was stuck at action_required at review time — get the full suite green on record before merging.