feat(desktop): add workspace rail unread observer

[wesbillman]

Summary

• add a Discord-style workspace rail on the far-left of the desktop shell
• add read-only inactive-workspace polling for rail unread state
• show dot for any unread activity and a numeric badge for mention count
• keep inactive observers isolated from the active relay singleton and active workspace caches
• cache the channel list per relay so switching workspaces paints the sidebar instantly (see "Instant channel-list paint" below)

Testing

pnpm --dir desktop typecheck
cd desktop && node --import ./test-loader.mjs --experimental-strip-types --test \
  src/features/workspaces/workspaceUnreadObserver.test.mjs \
  src/features/channels/readState/readStateManager.test.mjs \
  src/features/sidebar/ui/WorkspaceRail.test.mjs \
  src/features/channels/channelSnapshot.test.mjs
pnpm --dir desktop check:file-sizes

Full desktop suite green: just desktop-check, just desktop-typecheck, just desktop-test (1428/1428).

Rail e2e coverage added in desktop/tests/e2e/workspace-rail.spec.ts and validated by Brain one test at a time locally:

• rail shows a button per workspace and marks the active one
• clicking a workspace persists the active workspace id
• rail hides with a single workspace

Instant channel-list paint

Each workspace mounts a fresh React-Query client, so switching workspaces (or switching back to one just visited) started cold and blocked the sidebar on a multi-round-trip get_channels(). This adds a per-relay channel snapshot:

• On every successful getChannels(), the sorted channel list is persisted to localStorage keyed by normalized relay URL (channelSnapshot.ts, mirroring the existing selfProfileStorage pattern — versioned, tolerant reads/writes).
• useChannelsQuery seeds from that snapshot as initialData with initialDataUpdatedAt: 0, so the sidebar paints instantly with real data (names, types, unread) while the live fetch revalidates immediately in the background.
• Snapshots are GC'd on workspace removal, alongside the self-profile cache cleanup.

Scoped by relay URL, which matches today's workspace de-dupe (one relay = one workspace). If multiple identities on the same relay are ever supported, the key gains a :pubkey suffix — a one-line change.

All in desktop/src — no relay/DB/Rust changes. Covered by channelSnapshot.test.mjs (round-trip, key normalization, malformed/wrong-version tolerance, storage-failure tolerance).

Notes

Cross-relay unread/mention e2e is intentionally skipped for this PR because the current harness does not run two live relays. Observer behavior is covered by focused unit tests.

Screenshots to be uploaded in a follow-up PR comment.

[wesbillman]

Workspace rail — screenshots

Discord/Slack-style workspace rail on the far left. Shows here with 3 workspaces (Alpha active + highlighted, Bravo, Charlie) and the + add button.

Sidebar expanded

Sidebar collapsed — the rail stays pinned on the far left, fully visible and unshifted (fixes the reported bug where the rail's space was reserved but its buttons were pushed off-screen).

Unread dot + mention badge aren't visible in these shots (mock harness reports no unread); those render on ready state per the observer. Covered by the observer unit tests + workspaceRailIndicators unit tests.

Note: these reflect fix commit 6ce0b37e (rail moved to a sibling of SidebarProvider), which still needs to land on this PR branch — see the thread.