fix(tui): keep chat list sorted by updated.value across all mutations

[manuel-goepfi]

Summary

The chat list panel held the newest-first invariant only at first-shot load. Three mutation paths could subsequently desync the visible order from chat.conversation.updated.value, leaving stale chats stuck at the top of the list.

Root causes

• add_chat only sorted while ConversationsSnapshot.first_shot was True. After first-shot, a chat that appeared new to the widget map (because the panel had never seen it) jumped to position 0 regardless of timestamp.
• add_chat_at_top blindly prepended every chat passed to it, even when the chat's updated.value placed it elsewhere in the order.
• update_chat mutated title/summary in place but never re-positioned, so a chat whose updated.value just bumped (new message arrival) did not bubble to the top.

Fix

Centralise the ordering rule via two helpers (_chat_sort_key and its tuple variant) returning (rank, value) so chats with usable timestamps sort by datetime, while chats whose snapshot fails to load sink to the bottom under reverse=True without raising.

• load_chats sorts the incoming list before mounting (newest-first ordering even when copilot.chats() returns identifiers in arrival order).
• add_chat always sorts via _sort_items_by_updated. The existing _reorder_pending debounce moves into _schedule_reorder and continues to coalesce reorder passes during first-shot drain (avoids O(n²) when hundreds of WS-streamed chats arrive in one refresh cycle). Post-first-shot, events are sparse — reorder eagerly.
• add_chat_at_top becomes a thin shim over add_chat. A genuinely new chat lands at the top via its newest timestamp; a chat that merely appeared new to the widget map falls into its correct slot.
• update_chat resorts and reorders so a bumped updated.value bubbles the chat in the DOM to match the data invariant.
• base_list_panel._reorder_item_widgets narrows its bare-Exception handler to NoMatches (the only legitimately swallowable exception here — Textual mount is async); other exceptions propagate.

Sort failures are logged with chat id and exception type so a future regression that breaks .value loading is observable rather than silent.

Test plan

• Verified locally against 74 chats including a stale entry that previously stuck at position 0 — it now lands at the correct rank.
• No NameError on import (_chat_sort_key_tuple referenced before definition was the prior crash mode).
• First-shot drain still debounces reorders (_reorder_pending flag preserved).

[mark-at-pieces]

great change looking solid, might ass some basic unit tests;

but thinking updated is the way to go here!