feat(frontend/copilot): add chat search modal behind CHAT_SEARCH flag (#13139)

### Why / What / How

**Why:** Users with many chat sessions need a fast way to find a past
conversation without scrolling the sidebar.

**What:** Adds a chat search modal that lets users search across their
chat sessions by title, opened from the sidebar / mobile drawer or via
the `Cmd/Ctrl+K` shortcut. Behind the `CHAT_SEARCH` feature flag
(default off).

**How:** Client-side filtering for now — the modal receives the
already-fetched session list and filters/ranks it locally via
`useChatSearch` + helpers. Selecting a result routes to that session and
closes the modal. A backend-powered search endpoint will be wired up in
a follow-up PR (so content-level search and pagination can be
supported); this PR ships the UX shell behind a flag so it can be merged
independently.

> Note: this feature uses **client-side filtering only**. The backend
search integration will land in a follow-up PR.

### Changes 🏗️

- New `ChatSearchModal/` component: `ChatSearchModal.tsx`,
`ChatSearchResults.tsx`, `useChatSearch.ts`, `helpers.ts`
- `CHAT_SEARCH` feature flag added to `use-get-flag.ts` (default
`false`)
- `isSearchOpen` / `setSearchOpen` state added to `useCopilotUIStore`
(reset on logout)
- `ChatSidebar`: search button + `Cmd/Ctrl+K` keyboard shortcut to
toggle modal
- `MobileDrawer`: surfaces the search entry point on mobile
- Tests: store, ChatSidebar, MobileDrawer, ChatSearchModal helpers

### Checklist 📋

#### For code changes:
- [x] I have clearly listed my changes in the PR description
- [x] I have made a test plan
- [ ] I have tested my changes according to the test plan:
- [ ] Enable `CHAT_SEARCH` flag and confirm search button appears in
ChatSidebar (desktop)
- [ ] Press `Cmd+K` (mac) / `Ctrl+K` (win/linux) and confirm the modal
opens / closes
- [ ] Type a query and confirm results filter client-side by session
title
- [ ] Select a result and confirm it routes to that session and closes
the modal
- [ ] Open MobileDrawer and confirm search entry point is accessible on
mobile
- [ ] Disable the flag and confirm the search button + shortcut are
inert
  - [ ] Log out and confirm `isSearchOpen` resets

SECRT-2362