fix(mobile): protect unpublished channel-section edits and sync channel sort

[tellaho]

Problem

Mobile channel-section edits were silently lost ("groups don't persist"). Investigation on main (ab3af8287) showed:

1. The desktop channel-sections blob is on the relay and mobile decrypts/fetches/renders it correctly — the blank-sidebar screenshot was a stale build, not a read-path bug.
2. The real regression is on the write path: sections publish on a 5s debounce, but ChannelSectionsNotifier.build() tears the manager down on every relay-session status flip without flushing (dispose(flushPending: false)). The rebuilt manager fetches the stale remote blob and unconditionally adopts it over newer local state — the just-made edit vanishes.
3. Separately, mobile had zero channel-sort sync (channel-sort d-tag) — desktop-only since sync channel sort preferences across clients #1556.

Fix

Sections — dirty-state protection (mirrors the read-state manager pattern):

• Persisted dirty-since flag in ChannelSectionsStorage so unpublished edits survive manager teardown and app restarts.
• _mergeEvent never lets a remote blob overwrite unpublished local edits; the pending publish reconciles the relay (LWW timestamps still advance so our event sorts after the remote one).
• Publish-on-reconnect: initialize() re-schedules a publish when the dirty flag is set, and seed-publishes local state when the relay confirms it has no blob (never on a failed fetch).
• Edit-generation guard so a publish only clears the dirty flag when no new edit landed mid-flight.

Channel-sort parity (new mobile feature):

• channel_sort/ storage/manager/provider using the exact wire format desktop publishes: kind 30078, d-tag channel-sort, NIP-44 encrypted-to-self, whole-blob LWW, same group keys (starred/channels/dms/section:<id>, alpha/recent). Includes the same dirty-flag protection.
• Per-group Sort: Recent / A–Z menu on Starred, custom sections, Channels, and DMs; sort semantics match desktop's sortChannelsForSidebar (recent = newest first, message-less channels sink alphabetically). Orphaned section:<id> keys pruned on write.
• Offline local caching and encrypted relay semantics preserved throughout.

Verification

• 23 focused tests: teardown/rebuild clobber regression, publish-on-reconnect, dirty-flag persistence + clearing, seed-publish gating (incl. no-seed-on-failed-fetch), live-event-while-dirty, desktop wire-format round-trip, LWW adoption, orphan pruning, sort comparators.
• Full mobile suite: 588 passed, flutter analyze clean, dart format clean.
• Cross-impl NIP-44 decrypt of desktop (rust-nostr) ciphertext verified during investigation.