feat(gif): outlined GIF glyph, and GIF picker for the profile banner

The composer's GIF button drew a filled rounded rect with the letters knocked
out, which reads as a solid square rather than a picker. Invert it: stroked
outline with filled letters, reusing the original glyph paths scaled to centre.

Banners already accept absolute URLs on both ends (server isValidAssetUrl
allows http(s); the profile render branches on banner.startsWith('http')), so
the picker stores the remote URL directly with no upload path. Previews can now
hold either a blob: or an https: URL, so revoking is guarded — calling
revokeObjectURL on a remote URL is a silent no-op that would hide a mistake.

packages/web/src/components/chat/MessageInput.tsx

@@ -959,8 +959,14 @@ export function MessageInput({ channelId, channelName, placeholder }: MessageInp
               title="GIF"
               aria-label="GIF picker"
             >
-              <svg width="18" height="18" viewBox="0 0 24 24" fill="currentColor">
-                <path d="M2 5.5A2.5 2.5 0 0 1 4.5 3h15A2.5 2.5 0 0 1 22 5.5v13a2.5 2.5 0 0 1-2.5 2.5h-15A2.5 2.5 0 0 1 2 18.5v-13ZM5.1 14V10h3.2v1.2H6.5v.6h1.6v1.1H6.5V14H5.1Zm4.5 0V10h1.4v4H9.6Zm2.5 0V10h3.2v1.2h-1.8v.5h1.6v1h-1.6V14h-1.4Z" />
+              {/* Outlined badge, not a filled block: the solid rectangle read as
+                  a plain square rather than a GIF picker. Letters reuse the
+                  original glyph paths, scaled and centred inside the outline. */}
+              <svg width="20" height="20" viewBox="0 0 24 24" fill="none" aria-hidden="true">
+                <rect x="3" y="6" width="18" height="12" rx="3" stroke="currentColor" strokeWidth="2" />
+                <g fill="currentColor" transform="translate(-1.77 -4.2) scale(1.35)">
+                  <path d="M5.1 14V10h3.2v1.2H6.5v.6h1.6v1.1H6.5V14H5.1Zm4.5 0V10h1.4v4H9.6Zm2.5 0V10h3.2v1.2h-1.8v.5h1.6v1h-1.6V14h-1.4Z" />
+                </g>
               </svg>
             </button>
           )}