feat(channels): sort sidebar by latest activity, with favorite tier

Channels in the sidebar and mobile dropdown now sort by most recent
message first, with favorited channels pinned above non-favorites.
Reordering is push-driven via the existing new_message socket event:
the affected item is moved to the top of its tier in the DOM, no full
re-render. Favorites are toggled via a star icon in Manage Channels
and persisted in read_status.is_favorite for cross-device sync.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

app/schema.sql

@@ -164,6 +164,7 @@ CREATE TABLE IF NOT EXISTS read_status (
     key             TEXT PRIMARY KEY,       -- 'chan_0', 'dm_<pubkey>', etc.
     last_seen_ts    INTEGER DEFAULT 0,      -- unix timestamp
     is_muted        INTEGER DEFAULT 0,      -- 1 = muted (channels only)
+    is_favorite     INTEGER DEFAULT 0,      -- 1 = favorite (channels only)
     updated_at      TEXT NOT NULL DEFAULT (datetime('now'))
 );