#13: pin a message

Any participant can pin/unpin a message from its ⋮ menu. Adds pinned_at/pinned_by
columns, /api/messages/pin (toggle, broadcasts chat-pinned) and
/api/messages/pinned (list, newest first, excludes deleted + delete-for-me).
The conversation shows a pinned strip under the header (latest pin + count);
tap it to jump to the message, × to unpin. Live-updates across participants and
devices. Added pin/pinOff Lucide icons.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

server/public/icons.js

@@ -24,6 +24,8 @@
     search:      '<circle cx="11" cy="11" r="8"/><path d="m21 21-4.3-4.3"/>',
     edit:        '<path d="M12 20h9"/><path d="M16.5 3.5a2.12 2.12 0 0 1 3 3L7 19l-4 1 1-4Z"/>',
     trash:       '<path d="M3 6h18"/><path d="M19 6v14a2 2 0 0 1-2 2H7a2 2 0 0 1-2-2V6m3 0V4a2 2 0 0 1 2-2h4a2 2 0 0 1 2 2v2"/>',
+    pin:         '<path d="M12 17v5"/><path d="M9 10.76a2 2 0 0 1-1.11 1.79l-1.78.9A2 2 0 0 0 5 15.24V16a1 1 0 0 0 1 1h12a1 1 0 0 0 1-1v-.76a2 2 0 0 0-1.11-1.79l-1.78-.9A2 2 0 0 1 15 10.76V7a1 1 0 0 1 1-1 2 2 0 0 0 0-4H8a2 2 0 0 0 0 4 1 1 0 0 1 1 1z"/>',
+    pinOff:      '<path d="M12 17v5"/><path d="M15 9.34V7a1 1 0 0 1 1-1 2 2 0 0 0 0-4H7.89"/><path d="m2 2 20 20"/><path d="M9 9v1.76a2 2 0 0 1-1.11 1.79l-1.78.9A2 2 0 0 0 5 15.24V16a1 1 0 0 0 1 1h11"/>',
     logOut:      '<path d="M9 21H5a2 2 0 0 1-2-2V5a2 2 0 0 1 2-2h4"/><polyline points="16 17 21 12 16 7"/><line x1="21" x2="9" y1="12" y2="12"/>',
     plus:        '<path d="M5 12h14"/><path d="M12 5v14"/>',
     check:       '<path d="M20 6 9 17l-5-5"/>',