Adds a reasoning-effort quick-picker to the agent page's overflow menu,
a direct sibling of the existing model quick-picker, wired to the backend
seam from #1597/#1600: /api/state carries effort + available_efforts, the
picker POSTs { effort } to /api/effort (operator-only, mirroring
/api/model), and live updates arrive via the effort_changed SSE event.
The available levels are the backend's to declare — the frontend holds no
hard-coded list; availableEfforts is seeded from state.available_efforts
on cold-load and the picker section is omitted until the backend supplies
the set. Mirrors the model picker otherwise: postEffort(),
renderEffortChip() (updates the new amber effort-chip + button active
states), the cold-load seed, and the effort_changed handler. Build green.
Applies on the next claude session (operator uses the existing
clear-session), per the backend contract.
ceb3e3b01a