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/routers/frame_pages.py

@@ -25,6 +25,7 @@ from ..global_actions import GLOBAL_ACTION_LABELS
 from ..image_pipeline import (
     BORDER_STYLES,
     BORDER_STYLE_LABELS,
+    CALIBRATED_SPECTRA6_RGB,
     DEFAULT_PALETTE_RGB,
     DISPLAY_MODE_LABELS,
     MAX_BORDER_THICKNESS,
@@ -89,6 +90,7 @@ def frame_config_page(frame_id: int, request: Request, db: Session = Depends(get
         timezones=ALL_TIMEZONES,
         palette_labels=PALETTE_LABELS,
         default_palette_rgb=DEFAULT_PALETTE_RGB,
+        calibrated_spectra6_hex=palette_to_hex(CALIBRATED_SPECTRA6_RGB),
         palette_to_hex=palette_to_hex,
         photo_widget_id=photo_widget_id,
         global_action_labels=GLOBAL_ACTION_LABELS,