fix(frontend): resolve false error when moving chat to folder (#827)

Summary

Fixes the false "Failed to move conversation" error reported in #827 (closed). The move succeeded on the backend (HTTP 200, folderConversations edge created) but the frontend showed an error toast.

Root cause

1. Duplicate API call — ChatFolders.handleMoveChat() called chatHistoryService.moveConversation() directly and then dispatched the store action moveChat, which re-issued the same API call (the endpoint fired twice).

2. Cascading failure — chatHistoryStore.moveChat treated the post-move getFolder() refresh as part of the move transaction. A network/race failure there — or an unexpected folder shape (conversations undefined) — threw and propagated to the component's catch block, showing the error toast even though the move had already succeeded.

Changes

• ChatFolders.vue (handleMoveChat): removed the duplicate direct moveConversation() call. The store action now owns the API call + state sync.
• chatHistoryStore.js (moveChat):
  • commit('MOVE_CHAT', …) now runs immediately after the authoritative move succeeds, so local state reflects the move regardless of the folder refresh.
  • The getFolder() refresh is now best-effort (caught + logged) — a failure there no longer fails the move.
  • Null-guarded folder.conversations ((folder?.conversations || []).map(...)).
• chatHistory.test.js: two regression tests covering both failure modes.

Testing

• New regression tests fail against the unfixed code, pass after the fix.
• Full frontend Jest suite: 1203 passed, 0 failed (51 suites).
• ESLint + Prettier clean on all changed files.

Closes #827 (closed)