Add a curated theme system for "modern" style widgets, inspired by Tesserae

Frame.theme (7 presets in app/theme_tokens.py) drives font family,
corner radius, drop shadow, and an accent hue for every modern-style
widget's header/accent region. Rich accent colors (not just the 6 flat
panel inks) are approximated via denser Bayer stippling confined to just
that region (html_render.ordered_dither_regions), so icon/text content
elsewhere stays exactly as crisp as it is today -- verified directly
against real Chromium renders, both in unit tests and via run-server.
"classic" is a byte-identical no-visual-change default: weather's header
keeps its original fixed blue gradient, tasks/calendar keep their flat
THEME_* ink.

Themes are purely stylistic -- battery's charge-level color, calendar/
tasks' per-owner event chips, and text's own per-widget font choice are
never touched.

server/app/models.py

@@ -186,6 +186,11 @@ class Frame(Base):
     # for real photographs. NULL/1.0 = same defaults as the main fields.
     photo_palette_rgb: Mapped[list | None] = mapped_column(JSON, nullable=True, default=None)
     photo_dither_strength: Mapped[float] = mapped_column(Float, default=1.0)
+    # Curated visual theme for "modern" (HTML/CSS) style widgets -- see
+    # theme_tokens.THEMES. Frame-level (like palette_rgb/dither_strength
+    # above), not per-widget, since a theme is "how this frame looks."
+    # Widgets rendered in classic (PIL) style ignore this entirely.
+    theme: Mapped[str] = mapped_column(String, default="classic")
 
     # -- calendar mode (see calendar_feed.py, calendar_render.py,
     # routers/device.py's RENDERERS["calendar"]) --