Add Departures screen (train departure board)

New screens/departures.py using the free, unofficial db-vendo-client REST
API (v6.db.transport.rest) -- no key needed, same "no API key" philosophy
as Weather's Open-Meteo integration. Community-run infra though, not
official DB, and it was actually down (503, then timing out entirely) for
the whole time I was building this -- couldn't live-verify against real
data the way Open-Meteo was earlier, so I built the parsing against the
project's documented example response and stress-tested render() with
mocked requests.get() covering on-time/delayed/cancelled/missing-platform/
very-long-direction cases instead. check() fails soft (logs a warning,
returns None) on any request error, matching the "service might be down"
reality of this API -- the screen shows a plain "not available" message
rather than erroring out.

Settings: station name (resolved to a station ID via /locations, same
pattern as Weather's city->coordinates geocoding), how many departures to
show (3/5/8), and its own configurable update interval via the
get_poll_interval() mechanism built for Weather (defaults to 2 min --
departure boards go stale much faster than weather does).

Caught two real layout bugs while stress-testing render() before shipping:
line names ("ICE 1234") and delayed/cancelled time text ("Fällt aus") were
both wider than the columns I'd budgeted for them, with the second one
overlapping the direction text when right-aligned past its column's own
left edge. Fixed by measuring actual worst-case widths at the chosen font
size instead of guessing, and truncating every field to its column as a
defensive backstop regardless.

web/templates/base.html

@@ -185,6 +185,7 @@
       <a href="{{ url_for('index') }}" class="{{ 'active' if request.endpoint == 'index' }}">Status</a>
       <a href="{{ url_for('settings_navidrome') }}" class="{{ 'active' if request.endpoint == 'settings_navidrome' }}">Navidrome</a>
       <a href="{{ url_for('settings_weather') }}" class="{{ 'active' if request.endpoint == 'settings_weather' }}">Weather</a>
+      <a href="{{ url_for('settings_departures') }}" class="{{ 'active' if request.endpoint == 'settings_departures' }}">Departures</a>
       <a href="{{ url_for('settings_picture') }}" class="{{ 'active' if request.endpoint == 'settings_picture' }}">Picture</a>
       <a href="{{ url_for('settings_rotation') }}" class="{{ 'active' if request.endpoint == 'settings_rotation' }}">Rotation</a>
     </nav>