Add standalone weather widget (current/hourly/daily/multi-city, pluggable providers)

New widget type with four display modes -- current conditions, an
hourly forecast strip, a multi-day forecast, and several cities' current
day side by side -- backed by a pluggable provider registry (app/weather/,
mirroring the app/widgets/ dispatch pattern): Open-Meteo (worldwide) and
NWS (US-only) both wired up now, Environment Canada documented as the
next one to add given its more involved station/grid-lookup API.

The calendar widget's existing embedded weather strip is untouched and
still Open-Meteo-only; this lifts the same underlying icon-drawing
primitives (now shared via app/weather_render.py, calendar_render.py
still imports draw_weather_row unchanged) into a widget that can be
placed and sized on its own. Icons are redrawn in the panel's actual ink
colors (yellow sun/bolt, blue rain/snow) instead of flat black, and
build_multi_city's icon/font sizing now scales with how many cities need
to fit rather than the box's height alone -- both fixed after catching
them via live browser verification, along with a mode-switch cache-shape
crash and a mobile-width dialog overflow.

New WeatherWidgetConfig table (migration 24), grid footprint, widget
module, common.py fetch/cache helper, router endpoints (location set/
clear, city add/remove, preview), dialog template + JS, and full test
coverage (providers, widget render, HTTP endpoints, migration replay).
docs/widgets.md and CLAUDE.md's TODO updated accordingly.

CLAUDE.md

@@ -4,12 +4,12 @@ A DIY e-ink photo frame: an ESP32-C6 (`firmware/`, ESP-IDF) driving a
 Waveshare 7.3" E Ink Spectra 6 panel (800x480, 6-color, SPI), paired with a
 self-hosted FastAPI server (`server/`) that pulls from Immich, does all
 image processing (crop/dither/quantize/pack), and serves a placeable
-photos/calendar/whiteboard widget system to the device.
+photos/calendar/whiteboard/weather widget system to the device.
 
 CURRENT TODO
 -add more actions for buttons (i.e. change widget/layout)
 -Fix spurious button assignment stuff (probably but buttons on widget config with sane defaults)
--make a weather widget
+-add Environment Canada as a weather widget provider (app/weather/ -- station/grid-lookup API, more involved than Open-Meteo/NWS)
 -widget border option
 -battery life widget
 -sharing layouts with linked users