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.

README.md

@@ -1,8 +1,8 @@
 # Paperwall
 
 A standalone dashboard for one Waveshare `epd4in0e` (4", 6-color) e-ink panel:
-Navidrome now-playing, clock, weather, and pictures (upload or URL),
-switchable/controllable from a local web UI. Started as an InkyPi
+Navidrome now-playing, clock, weather, train departures, and pictures
+(upload or URL), switchable/controllable from a local web UI. Started as an InkyPi
 replacement, but doesn't depend on InkyPi at all -- own venv, own vendored
 copy of the Waveshare driver files.