Clock: a "Delphin Uhr" overlay with the school bus and a parents' timer

Einfg on the kiosk keyboard, or Guide (KEY_EPG) on the IR remote, shows a big
analog watch over whatever page is open, built from the Delphin Uhr mockup
without its swimming dolphin. The overlay stays mounted and is only hidden, so it
appears at once. Beside the watch, only while they run:

- The bus: on school days, a silent countdown to `general.clock.bus_time` (07:25)
  that starts `bus_lead_minutes` (60) before. School days skip weekends and the
  school and public holidays of `general.clock.subdivision` (DE-BY), fetched daily
  from openholidaysapi.org and cached under the library cache. A badge on the
  clock says whether there is school today, and why not.
- The dolphin timer, set in parent mode (5/10/15/30 minutes, or until a time).
  It lives in the backend, so a parent's phone can start it for the kiosk. When it
  runs out the kiosk opens the clock at 00:00 and a soft bell chimes on the
  device; it clears after a minute, or at the next key, tap or remote press.

Parent mode is now a way in rather than a page: `?parentMode=1` adds a 🔧 button
next to the ⌨ one, or an "Eltern" tab in the phone's dock, so the URL can stay
bookmarked. The timer comes first there, the device settings below it.

Unmapped IR buttons are logged at debug level, to find out what irserver calls a
new one.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

python-backend/README.md

@@ -212,8 +212,8 @@ copied to the device.
 | `GET /api/library` | Every album with its tracks and colours. ~90 kB, sent once. |
 | `GET /api/albums/{id}/cover` | The cover, or 404 - the client paints the album's colours instead. |
 | `POST /api/library/refresh` | Rescan in the background; connected clients are told when it lands. |
-| `GET /api/state` | Snapshot: what is playing, where, how loud. |
-| `WS /api/ws` | Push only. A snapshot on connect, then a frame per change, plus the position at 2 Hz while playing. |
+| `GET /api/state` | Snapshot: what is playing, where, how loud; plus `clock` - today's school day, the bus, the parents' timer. |
+| `WS /api/ws` | Push only. A snapshot on connect, then a frame per change, plus the position at 2 Hz while playing, and `clock_toggle` when the remote's Guide key asks for the clock. |
 | `POST /api/play` | `{album_id, track_index?}` |
 | `POST /api/resume` `/pause` `/next` `/previous` `/seek` | transport |
 | `POST /api/volume` | `{percent}`, `{delta_percent}`, or `{steps}` - the IR remote's step (`volume_increment` device units) per step |
@@ -221,6 +221,7 @@ copied to the device.
 | `GET` `PUT /api/remote/mapping` | number key -> album (optionally from one `track_index`) or podcast show |
 | `POST /api/screen/toggle` `/screen/activity` | the kiosk's Del key; its key presses and taps, holding off the idle screen-off |
 | `GET` `PUT /api/settings` | parent mode |
+| `POST` `DELETE /api/timer` | parent mode's countdown: `{minutes}` or `{until: "HH:MM"}`; stop or silence it |
 
 Two decisions worth knowing: