fix(workspaces): exclude muted channels and unfollowed threads from rail unread badge

[wpfleger96]

Problem

Two classes of events were incorrectly lighting the workspace rail's unread dot:

1. Muted channels — channels the user has muted still counted toward hasUnread because fetchObservedChannels filtered archived channels and hidden DMs but not muted channels.
2. Unfollowed thread replies — threaded replies in threads the user was not following, participating in, authored, or mentioned in lit the rail dot, even though the in-workspace sidebar dot correctly ignores them.

When the user switched into the relay, both badges disappeared — the two code paths disagreed on what counts as unread.

Root cause

fetchWorkspaceUnread in workspaceUnreadObserver.ts:

• Never checked channel mutes (kind 30078, d=channel-mutes)
• Set hasUnread on any external unread event via isUnreadExternalEvent, with no thread-relevance gate

The in-workspace sidebar uses shouldNotifyForEvent (a pure function in shouldNotify.ts) to gate events through the same thread-relationship sets. The rail observer did not.

Fix

Commit 1 — muted channels:
Fetch the channel-mutes blob (kind 30078, d=channel-mutes, nip44-self-encrypted) alongside read-state in a single Promise.all, decrypt via nip44DecryptFromSelf, derive the muted id set with mutedChannelIdsFromStore, and continue past muted channels in the loop.

Commit 2 — thread-relevance gate:
Before the per-channel loop, read the four by-pubkey localStorage sets — participated (buzz-thread-participation.v1), followed (buzz-thread-follows.v1), authored (buzz-thread-authored.v1), muted-roots (buzz-thread-muted.v1) — and gate each candidate unread event through shouldNotifyForEvent in addition to the existing isUnreadExternalEvent check. The follows set uses ThreadFollowEntry[] format so it gets a thin dedicated reader.

Both injectables follow the existing decryptReadState/decryptMutes pattern for unit-testability.

Tests

14 tests total (9 pre-existing + 5 new for thread-relevance gate):

• Muted-only channel → {hasUnread: false, mentionCount: 0}
• Mixed muted+unmuted — only unmuted channel counts
• Decryption failure → treated as empty mutes (channel still counted)
• Absent mutes blob → treated as empty mutes
• Threaded reply in untracked root → hasUnread: false
• Same reply with root in participatedRootIds → hasUnread: true
• #p-mention reply in untracked root → hasUnread: true (mention overrides gate)
• Top-level post → hasUnread: true (no thread gate for top-level)
• Threaded reply whose root is in mutedRootIds → hasUnread: false (mute wins even if participated)

tsc --noEmit clean, pnpm build clean.