feat(dm): "Hidden conversations" sub-menu to un-hide DM threads

[sanity]

Problem

PR #265 (issue #261) implemented the local-only hide-stale-DM-threads filter for the left rail. The original issue marked a "Hidden conversations" sub-menu as optional for v1, so it was deferred — but it should be tracked.

Today, a thread can be un-hidden only by:

1. Sending an outbound DM to the peer (calls unhide_dm_thread via the Codex P1 fix).
2. Receiving a new inbound DM from the peer (strict > on last_any_ts revives the thread via the filter).

There is no UI path to un-hide a thread if the user just changed their mind. The hidden entry persists across reloads (it rides OUTBOUND_DMS_STORAGE_KEY) and across devices, so a misclick on Hide is recoverable only via one of the two implicit paths above.

Approach

The infrastructure is already in place:

• unhide_dm_thread(room_owner_vk, peer) in ui/src/components/app/chat_delegate.rs is implemented, tombstoned-against-late-hydration (Codex P3), and unit-tested.
• HIDDEN_DM_THREADS is a GlobalSignal<HashMap<(VerifyingKey, MemberId), HiddenDmThreadEntry>> so a submenu can iterate it directly.
• The HiddenDmThreadEntry carries room_owner_vk + peer + hidden_at_ts. Resolving the room display name + peer nickname goes through the same path DmRailSection uses.

Suggested UX (open to revision):

• Settings → "Hidden conversations" panel (or kebab menu in the DM rail header).
• One row per hidden entry, showing room name + peer nickname + relative "hidden N days ago".
• Click "Show again" → unhide_dm_thread(...) and the thread reappears in the rail.

Scope notes

• v1 deliberately shipped without this submenu (per the original feat: hide stale DM threads from the left rail (local-only) #261 wording — "optional"). This issue is to track that v1 follow-up, not block feat: hide stale DM threads from left rail (#261) #265.
• Keep the panel local-only — no contract changes, no second delegate storage key.
• The same legacy-migration probe set continues to apply because hides ride OUTBOUND_DMS_STORAGE_KEY (Phase 6 note in AGENTS.md).

References

• feat: hide stale DM threads from the left rail (local-only) #261 — the original "hide stale DM threads" issue.
• PR feat: hide stale DM threads from left rail (#261) #265 — implementation.
• ui/src/components/app/chat_delegate.rs — hide_dm_thread, unhide_dm_thread, decide_hide_action.

[AI-assisted - Claude]