修复 #3：以手动快照同步替代微信增量轮询

[NytePlus]

Closes #3.

wechat_get_updates previously polled an archive that never imported new messages after the initial load. Replace that behavior with explicit wechat_sync(conversation) and a 手动同步 button on each approved conversation. Approval leaves a new conversation waiting for its first manual sync; query tools read the resulting archive without polling.

The pinned backend reads a finite snapshot from each source shard and resumes saved rowid positions, deferring subsequent arrivals until the next sync. It commits messages and cursors together, includes late/backdated newly appended rows and discovers new shards. Every archive read, including empty results and media tool responses, reports last_synced_at (null until first successful sync); lists/status carry the time per conversation. Failed syncs leave the time unchanged. Archive-only updates do not suggest a live source check. The obsolete session-index readiness error no longer blocks manual sync.

• Backend dependency: Add manual snapshot sync for iMCP conversations wx-cli#1
• Validation: native macOS Xcode Debug build succeeded; 132 Rust unit tests passed (including the latest 12 backend tests); diff whitespace checks passed. Docker daemon was unavailable. Signed release installed and process launch verified; real-account sync and full UI acceptance were not exercised.
• Complexity: O(Δ) message visits/decoding, plus shard discovery and indexed seek/write/FTS overhead; no historical message scan. Full details are in Docs/WeChat/IMPLEMENTATION.md.
• Boundaries: snapshots are per database file, not a globally atomic instant; first-observed append semantics do not detect in-place edits or reused rowids. Initial history imports have a 300-second client timeout and roll back on termination.

WeChat settings use the positive “图片配置可用” status (requiring a configured image key and no config error). Remove the “WeChat 自用构建 · 自动更新已关闭” text below Settings.