feat: add persistent thread rename and sidebar "Rename" action

[Dimillian]

Motivation

• Users need to assign meaningful names to threads instead of the auto-generated Agent X/ID-based names to easily identify threads by topic.
• Custom names must persist across restarts and survive listing/pagination/resume flows without being clobbered by streaming updates.
• Avoid passing external storage state into the reducer by using a heuristic to protect custom names from automated renames.

Description

• Added localStorage-backed custom name persistence with caching: introduced STORAGE_KEY_CUSTOM_NAMES, makeCustomNameKey, loadCustomNames, saveCustomName, and a cached customNamesRef + getCustomName in useThreads to avoid repeated reads. (file: src/features/threads/hooks/useThreads.ts)
• Applied custom-name priority in thread name resolution across listThreadsForWorkspace, loadOlderThreadsForWorkspace, and resumeThreadForWorkspace so cached custom names are used before backend preview or fallback names; and skip setting preview-based name when a custom name exists. (file: src/features/threads/hooks/useThreads.ts)
• Exposed a renameThread callback from useThreads that writes to localStorage, updates the cache, and dispatches setThreadName. (file: src/features/threads/hooks/useThreads.ts)
• Added a Rename menu item to the thread context menu and wired a prompt-based handler in App.tsx that calls renameThread; passed handler through useSidebarMenus -> Sidebar -> layout nodes. (files: src/features/app/hooks/useSidebarMenus.ts, src/features/app/components/Sidebar.tsx, src/features/layout/hooks/useLayoutNodes.tsx, src/App.tsx)
• Heuristic guard to protect custom names: added looksAutoGeneratedThreadName and used it in maybeRenameThreadFromAgent and the inline rename logic so auto-renames only run when the current name appears auto-generated. (file: src/features/threads/hooks/useThreadsReducer.ts)
• Key details: storage keys use workspaceId:threadId to avoid collisions across workspaces, parsing/writes are best-effort (try/catch) and invalid JSON is ignored.

Testing

• Ran npm run lint which completed (note: eslint emitted a TypeScript-version warning about TS 5.8.3 vs supported range).
• Ran npm run typecheck (tsc --noEmit) and it completed without errors.
• No UI screenshot tests were run in this environment; manual UX behaviors expected by the change are documented in the PR checklist (prompt, persistence, and rename protection).

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[Dimillian]

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