Add link previews, voice tiles and a menu volume slider; tidy the profile

- Link previews for web links in chat, fetched by the f0ckm upload host by
  default (linked sites never see the user's IP), or directly, or off
- Voice channel tiles above the chat on windows too narrow for the member list
- Per-person volume slider in the user context menu
- Profile panel: channel on its own line under the name, no Guest badge,
  voice actions moved to the context menu
- User information dialog refreshes every second
- Rate limiting mirrors Murmur's whole-second bucket leak, so irregular
  bursts are queued instead of dropped
- CHANGELOG.md

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/lib/menu.svelte.ts

@@ -5,6 +5,8 @@ export interface MenuItem {
   checked?: boolean;
   disabled?: boolean;
   separator?: boolean;
+  // Inline slider instead of a clickable item; the menu stays open while dragging
+  slider?: { value: number; min: number; max: number; step: number; format: (v: number) => string; oninput: (v: number) => void };
 }
 
 // One context menu for the whole app, opened at the pointer position