Roll out "modern" HTML/CSS render style to every widget except photos

Extends weather's experimental Chromium+Jinja2 render style to battery,
text, tasks, static image, whiteboard, and calendar (all four view
modes -- agenda/today_tomorrow/week/month), and gives the photos widget
its own genuinely independent palette + dithering strength.

Photos: Frame.photo_palette_rgb/photo_dither_strength (mirroring the
existing palette_rgb/dither_strength), with a second "Photos
configuration" card in Advanced Configuration. widgets/photos.py's
render() quantizes itself against these before returning -- no
render_panel changes needed, since photos is the only widget that
genuinely needs a different reference palette and can carry that
itself, the same way modern-style widgets already self-dither via
ordered_dither.

Battery/text/tasks/static image/whiteboard: same render_style pattern
weather established (render_style column, html_render.py build
function, Jinja2 template, dialog toggle). Static image/whiteboard get
their first-ever visual chrome (a rounded-corner shadowed card,
shared framed_image.html.jinja) since classic draws them with zero
frame at all. Fixed the same "preview endpoint bypasses render_style"
bug weather originally shipped with, for tasks/static/whiteboard/
calendar's preview endpoints.

Calendar: own module (app/calendar_html_render.py, mirroring
calendar_render.py's separation from the simpler widgets) covering all
four view modes, not just agenda -- reuses calendar_render's own
private helpers so event colors/times/weather/month-grid math match
classic exactly. Found and fixed two real cross-day layout bugs along
the way: a per-day header height that varied based on whether that
specific day had a weather entry (misaligning where every other day's
event rows started across the week/month grid), and regular-weight
small text being fragile under Bayer ordered dithering (out-of-month
day numbers degraded into unrecognizable speckle) -- fixed by using
bold everywhere and de-emphasizing via size instead of weight/gray,
since gray text has the same dithering fragility this project's PIL
renderers already avoid for exactly this reason.

Migrations 32-38 (Frame's two new columns, then one render_style column
per widget config table). 452 tests passing, including new dispatch/
migration coverage per widget type and a dedicated photos test proving
photo_palette_rgb produces genuinely independent quantization from the
frame's main palette_rgb.

server/app/routers/api_frames.py

@@ -105,6 +105,9 @@ def api_config_save(
     color_boost: float | None = Form(None),
     contrast_boost: float | None = Form(None),
     dither_strength: float | None = Form(None),
+    photo_palette: list[str] | None = Form(None),
+    photo_palette_reset: bool | None = Form(None),
+    photo_dither_strength: float | None = Form(None),
     hold_duration_ms: int | None = Form(None),
     next_hold_action: str | None = Form(None),
     back_hold_action: str | None = Form(None),
@@ -182,6 +185,17 @@ def api_config_save(
             cfg.contrast_boost = max(0.0, min(2.0, contrast_boost))
         if dither_strength is not None:
             cfg.dither_strength = max(0.0, min(1.0, dither_strength))
+        if photo_palette_reset:
+            cfg.photo_palette_rgb = None
+        elif photo_palette is not None:
+            if len(photo_palette) != len(PALETTE_LABELS):
+                raise HTTPException(400, f"Expected {len(PALETTE_LABELS)} palette colors, got {len(photo_palette)}")
+            parsed = [hex_to_rgb(h) for h in photo_palette]
+            if any(rgb is None for rgb in parsed):
+                raise HTTPException(400, "Palette colors must be #rrggbb hex values")
+            cfg.photo_palette_rgb = [list(rgb) for rgb in parsed]
+        if photo_dither_strength is not None:
+            cfg.photo_dither_strength = max(0.0, min(1.0, photo_dither_strength))
         if hold_duration_ms is not None:
             cfg.hold_duration_ms = max(MIN_HOLD_DURATION_MS, min(MAX_HOLD_DURATION_MS, hold_duration_ms))
         if next_hold_action is not None: