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/widgets/tasks.py

@@ -43,7 +43,7 @@ def render(db: Session, frame: Frame, widget: Widget, target_w: int, target_h: i
         # own classic path, should never pay for it.
         from .. import html_render
 
-        return html_render.build_tasks(tasks, target_w, target_h, frame.palette_rgb, title)
+        return html_render.build_tasks(tasks, target_w, target_h, frame.palette_rgb, title, frame.theme)
     return _build_tasks(tasks, target_w, target_h, frame.palette_rgb, title)