Phase 4: power UX (palette, shortcuts, mentions, mobile, PWA)

Command palette (cmd+K) with fuzzy nav, ticket search, people lookup
and action entries (new ticket, logout, show shortcuts). Opens from
keyboard or user dropdown.

Global keyboard shortcuts via a small useShortcut/useLeaderShortcut
hook: `?` help overlay, `c` new ticket, `g d|t|m|n|s` leader nav.
Tickets list: j/k cursor, Enter open, x toggle select. TicketDetail:
`e` edit, `r` focus comment composer. All guarded against firing
inside text fields.

@mention autocomplete in the comment composer (MentionTextarea) with
arrow-key nav and Tab/Enter insert. Rendered comments and audit log
rewrite @username tokens to links pointing at that user's assignee
filter; unknown usernames left as plain text.

Mobile sweep: TicketDetail sidebar stacks below content on <md,
Settings profile grid collapses to one column, admin tables get
horizontal scroll with a 640px min width, CTI 3-column grid stacks
vertically on <md, New ticket severity/assignee grid same.

PWA: manifest.webmanifest, SVG icon, minimal network-first service
worker for the app shell (never caches /api/*), registered in
production builds only. Theme-color meta + manifest link in index.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

client/src/pages/Settings.tsx

@@ -59,7 +59,7 @@ export default function Settings() {
         {/* Profile */}
         <section className="rounded-md border border-border p-4">
           <h2 className="text-sm font-semibold mb-3">Profile</h2>
-          <div className="grid grid-cols-2 gap-3 text-sm">
+          <div className="grid grid-cols-1 sm:grid-cols-2 gap-3 text-sm">
             <Field label="Display name" value={user?.displayName ?? ''} />
             <Field label="Username" value={user?.username ?? ''} mono />
             <Field label="Email" value={user?.email ?? ''} />