Add experimental HTML/CSS "modern" render style for weather widget

The weather widget's icons/layout are hand-drawn PIL primitives -- clean
under quantization but flat, no gradients/shadows. Adds an opt-in
render_style="modern" (current/daily modes only) that instead renders a
Jinja2 template through a persistent headless-Chromium browser
(app/html_render.py), following the approach of Tesserae, an open-source
e-ink dashboard targeting this same panel family.

Key design points:
- The Chromium dependency (Playwright) is lazily imported only when a
  weather widget actually uses "modern" style, and the background browser
  itself only launches on first use -- every other widget type, and this
  one's own classic/hourly/multi_city paths, never pay for it.
- No Frame-level dithering setting needed: html_render dithers its own
  rendered widget to exact palette colors (Bayer/ordered, not
  Floyd-Steinberg) before compositing, so the shared whole-canvas
  Floyd-Steinberg pass sees zero quantization error there and leaves it
  untouched -- same trick draw_text/hand-drawn icons already use. Floyd-
  Steinberg keeps working unchanged for photos and every other widget.
- A "Load calibrated Spectra 6 preset" button in Advanced configuration
  offers a community-measured palette (data ported from
  paperlesspaper/epdoptimize, Apache 2.0) as an alternative starting
  point to the existing idealized DEFAULT_PALETTE_RGB -- fills the
  existing palette table, doesn't save by itself.

Known open risk, not resolved here: a headless Chromium binary is far
larger than the ~100MB single-layer limit that already forced this
project's pip/npm installs into split layers, and (unlike those) is a
single ~180MB file that can't be split across layers by ordinary
Dockerfile restructuring. Flagged prominently in server/Dockerfile and
docs/widgets.md -- treat this render style as experimental/local-only
until that's resolved.

docs/widgets.md

@@ -269,6 +269,25 @@ modes (`WeatherWidgetConfig.mode`, switchable in the widget's dialog like
   side -- the calendar widget's embedded strip, as a standalone
   widget's whole content instead of a strip above an agenda day.
 
+**Render style** (`WeatherWidgetConfig.render_style`, `"classic"` default
+| `"modern"`, experimental): `current`/`daily` only -- `hourly`/
+`multi_city` always render classic regardless of this setting. `modern`
+draws the widget as an HTML/CSS card (Jinja2 templates under
+`app/templates/widget_html/`) through a persistent headless Chromium
+browser (`app/html_render.py`, Playwright) instead of `app/weather_render.
+py`'s hand-drawn PIL primitives -- gradients/shadows/soft icon shading
+PIL can't easily do. Its own `ordered_dither` (Bayer/ordered, not Floyd-
+Steinberg) commits the rendered widget to exact palette colors *before*
+compositing, so it's safe to mix with photo/other classic-rendered
+widgets on the same frame without a Floyd-Steinberg seam at the boundary
+(see that module's docstring for why ordered dithering doesn't have this
+problem and Floyd-Steinberg does) -- no `Frame`-level dithering setting
+was needed. Playwright/Chromium is a real, heavyweight runtime dependency
+imported lazily only when a weather widget actually uses this style, and
+its Docker packaging has a known likely-blocking image-size problem not
+yet resolved (see `server/Dockerfile`'s own comment) -- treat this style
+as unshipped/local-only until that's sorted out.
+
 `current`/`hourly`/`daily` share one configured location
 (`city_label`/`city_latitude`/`city_longitude`, set via `POST .../
 weather-location`, geocoded through `weather.geocode_city`); `multi_city`