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.

server/app/models.py

@@ -607,13 +607,19 @@ class WeatherWidgetConfig(Base):
     `cached`'s shape depends on `mode`: {"temp","category"} for current,
     a list of {"time","temp","category"} for hourly, a
     {"YYYY-MM-DD": {...}} dict for daily, or a list of
-    {"label","high","low","category"} for multi_city."""
+    {"label","high","low","category"} for multi_city.
+    `render_style` picks which renderer draws the widget: "classic" (the
+    hand-drawn PIL primitives in app/weather_render.py, unchanged
+    default) or "modern" (app/html_render.py's Jinja2/headless-Chromium
+    path, "current"/"daily" modes only for now -- see weather.py's
+    render())."""
 
     __tablename__ = "weather_widget_configs"
 
     widget_id: Mapped[int] = mapped_column(ForeignKey("widgets.id", ondelete="CASCADE"), primary_key=True)
     mode: Mapped[str] = mapped_column(String, default="current")  # current | hourly | daily | multi_city
     provider: Mapped[str] = mapped_column(String, default="open_meteo")  # open_meteo | nws
+    render_style: Mapped[str] = mapped_column(String, default="classic")  # classic | modern
     units: Mapped[str] = mapped_column(String, default="fahrenheit")  # fahrenheit | celsius
     # Single-location modes only (current/hourly/daily) -- geocoded once
     # via weather.geocode_city() when set, same idiom as