#19895 cmux-next: chats lists say chats; no-match message once

The chats lists now call chats "chats". A search that matches nothing says so once.

• The chats sidebar said "No sessions yet" and "No matching sessions", while the same list says "chats" everywhere else (New Chat, "No settled chats yet", New Tab's Chats). These now read "No chats yet" and "No matching chats". New Tab's Chats section also shared the search wording, and its "No chats" is now "No chats yet".
• The search field's accessibility label said "Search sessions"; it now says "Search chats" (sidebar and New Tab's All Chats).
• With a query that matches nothing, "No matching sessions" showed at the top and again under an empty Settled section. Settled now stays hidden while the top line says nothing matches.

Polish sweep (empty states).

Tests

webviews/test/chats-sidebar-empty.test.tsx renders the sidebar, searches, and counts the message. Red on 3927afe1c104: the message appeared 2 times, and "No sessions yet" showed. Green on the head. The search-label check is red on 79facf8783d7 (`Search sessions`) and green on the head. bun run check passes.

Localization

All 21 locales were updated in acpmux/Localizable.xcstrings, using each locale's existing word for "chat" from newTab.noChats. generated/strings.json was regenerated with gen-strings.mjs agentPane.

Changelog

• Changed: the chats sidebar says "chats" instead of "sessions", and a search with no matches says so once.

🤖 Generated with Claude Code