feat: channel permissions UI, DM Sans font, private channel filtering, migration fix

- Rewrite ChannelSettingsModal with full tri-state permission override UI
  for roles and members (allow/neutral/deny per permission bit)
- Switch font from Inter to self-hosted DM Sans (woff2 variable fonts)
- Add client-side VIEW_CHANNEL filtering in ChannelSidebar for private channels
- Broadcast isPrivate flag on channel override changes
- Fix voice permission bit migration: gate behind persistent flag to prevent
  repeated re-runs that stripped STREAM from @everyone roles
- Add speakingUserIds set to voice store for efficient user-level lookups
- Clear current channel view when a channel is deleted
- Move .glass-strip to @layer utilities for proper CSS specificity
- Simplify avatar initials font size to proportional formula

packages/web/tailwind.config.js

@@ -70,7 +70,7 @@ export default {
         'glass': '0 2px 8px rgba(0,0,0,0.25), 0 8px 24px rgba(0,0,0,0.15), inset 0 1px 0 var(--glass-highlight)',
       },
       fontFamily: {
-        sans: ['Inter', '-apple-system', 'BlinkMacSystemFont', 'system-ui', 'sans-serif'],
+        sans: ['DM Sans', '-apple-system', 'BlinkMacSystemFont', 'system-ui', 'sans-serif'],
       },
     },
   },