feat(frontend/copilot): paginate session sidebar so threads past 50 stay reachable (#13128)

### Why / What / How

**Why.** The CoPilot session sidebar fetched only the first 50 sessions
and stopped. Any user with more than 50 chats lost access to every older
thread through the UI even though the backend already supported `offset`
and returned a `total` count. Threads weren't lost in the DB — they were
just invisible.

**What.** Wire proper pagination into the sidebar and mobile drawer with
a "Load older chats" affordance, backed by a single shared
infinite-query hook. Reroute all session-list cache invalidations so
they refresh every loaded page, not just page 1.

**How.**
- New `useSessionList` hook wraps `useInfiniteQuery` around the existing
`getV2ListSessions` fetcher: page size 50, `offset` pageParam,
`refetchInterval` 10s, `getNextPageParam` derived from `total`.
- Hook lives on a fresh `SESSION_LIST_QUERY_KEY` so the infinite cache
doesn't collide with the orval-generated single-query key shape (which
would break shape-sensitive cache readers).
- `ChatSidebar` and `MobileDrawer` consume the hook and render a ghost
`Button` with `loading` state at the end of the list when `hasMore`.
- `useSessionTitlePoll` walks `InfiniteData` pages via a small
`flattenSessions` helper instead of reading the legacy single-page
response.
- Five invalidation callsites (`useChatSession`,
`useCopilotNotifications`, `useSessionDeletion`, `useSessionTitlePoll`,
`ChatSidebar`) updated to invalidate the new key, so
create/delete/rename/cross-tab-sync still refresh the sidebar correctly.

### Changes 🏗️

- Add `frontend/src/app/(platform)/copilot/useSessionList.ts` (shared
paginated hook + `SESSION_LIST_QUERY_KEY` + `flattenSessions` cache
walker).
- `ChatSidebar.tsx`: replace single-page `useGetV2ListSessions` with
`useSessionList`, add Load-more button, point invalidations at the new
key.
- `MobileDrawer.tsx`: mirror the same paginated fetch + Load-more
affordance.
- `useSessionTitlePoll.ts`: walk infinite-cache pages; invalidate the
new key.
- `useChatSession.ts`, `useCopilotNotifications.ts`,
`useSessionDeletion.ts`: switch invalidation calls to
`SESSION_LIST_QUERY_KEY`.

### 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:
  - [x] `pnpm types` — clean
  - [x] `pnpm lint` — clean (only pre-existing img-tag warnings)
- [x] `pnpm test:unit src/app/(platform)/copilot` — 59 files / 929 tests
pass, including the existing `ChatSidebar` delete + status-indicator
suites against the new wiring
- [ ] Manual: seed a user with >50 sessions; confirm "Load older chats"
appears, paginates correctly, and that the button disappears when
`loaded === total`
- [ ] Manual: confirm the 10s refetch still surfaces
running/queued/processing indicators on all loaded pages
- [ ] Manual: delete / rename / create-new-session — confirm the sidebar
refreshes
- [ ] Manual: cross-tab `localStorage` storage event still invalidates
the list
- [ ] Manual: title-poll still animates the new title in after stream
completion

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Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>