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/shared/src/types.ts

@@ -149,6 +149,7 @@ export interface Channel {
   topic: string | null;
   position: number;
   categoryId: string | null;
+  isPrivate?: boolean;
   createdAt: number;
   lastMessageId?: string | null;
   myPermissions?: string; // Computed per-user BigInt decimal string