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/tests/test_widgets_calendar.py

@@ -9,6 +9,8 @@ from __future__ import annotations
 
 import time
 
+import pytest
+
 from app import widgets
 from app.db import widget_locked
 from app.models import CalendarWidgetConfig, Frame, Widget
@@ -163,3 +165,46 @@ def test_month_view_falls_back_to_agenda_layout_below_small_tier(db_session, mon
     _stub_fetches(monkeypatch)
     img = widgets.calendar.render(db_session, frame, widget, 300, 192)
     assert img.size == (300, 192)
+
+
+# --- "modern" style (app/calendar_html_render.py) -----------------------
+# No real browser here -- html_render.render_html_to_image is monkeypatched
+# to a stub, so these exercise calendar.py's dispatch + calendar_html_
+# render's own layout/data logic, not Playwright/Chromium itself.
+
+def _stub_render_html_to_image(monkeypatch):
+    from PIL import Image
+
+    from app import html_render
+
+    monkeypatch.setattr(html_render, "render_html_to_image",
+                        lambda html, target_w, target_h: Image.new("RGB", (target_w, target_h), (255, 255, 255)))
+
+
+@pytest.mark.parametrize("view", ["agenda", "today_tomorrow", "week", "month"])
+def test_render_modern_style_every_view(db_session, monkeypatch, view):
+    """All four view modes have a modern-style builder (unlike weather's
+    own current/daily-only modern style) -- each must dispatch correctly
+    from calendar.py's render()."""
+    frame, widget = _make_widget(db_session, view=view, render_style="modern")
+    events = [{"summary": "Standup", "start": "2026-07-31T09:00:00+00:00",
+              "end": "2026-07-31T09:30:00+00:00", "all_day": False,
+              "sources": [{"owner_display_name": "Alice", "color_index": None}]}]
+    _stub_fetches(monkeypatch, events=events)
+    _stub_render_html_to_image(monkeypatch)
+
+    img = widgets.calendar.render(db_session, frame, widget, 380, 300)
+    assert img.size == (380, 300)
+    assert img.mode == "RGB"
+
+
+def test_render_modern_month_falls_back_to_agenda_below_small_tier(db_session, monkeypatch):
+    """Same "month needs real column width" fallback classic has (see
+    test_month_view_falls_back_to_agenda_layout_below_small_tier above),
+    still honored when render_style is modern."""
+    frame, widget = _make_widget(db_session, view="month", render_style="modern")
+    _stub_fetches(monkeypatch)
+    _stub_render_html_to_image(monkeypatch)
+
+    img = widgets.calendar.render(db_session, frame, widget, 300, 192)
+    assert img.size == (300, 192)