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/tests/test_widget_config_and_queue_endpoints.py

@@ -243,6 +243,35 @@ def test_config_save_switching_mode_clears_the_now_incompatible_cache(client, db
     assert resp.status_code == 200, resp.text
 
 
+def test_preview_weather_honors_modern_render_style(client, db_session, monkeypatch):
+    """Regression test: api_widget_preview_weather originally called
+    weather_render.render_weather_preview_png directly, unconditionally --
+    the dialog's own live preview never reflected render_style="modern" at
+    all, even though the real device-facing render (widgets/weather.py's
+    render()) did. Route through html_render instead for modern/current or
+    modern/daily, same as the device path -- assert it's actually reached,
+    not just that the request 200s (it would 200 either way if this
+    silently fell back to classic)."""
+    client.post("/setup", data={"username": "alice", "password": "hunter22"})
+    widget = _add_weather_widget(db_session, mode="current", render_style="modern")
+    monkeypatch.setattr(
+        "app.routers.api_widgets.get_or_refresh_weather_widget_data",
+        lambda db, frame, widget, force=False: {"temp": 72.0, "category": "clear"},
+    )
+    calls = []
+
+    def _fake_render_html_to_image(html, target_w, target_h):
+        calls.append((target_w, target_h))
+        return Image.new("RGB", (target_w, target_h), (255, 255, 255))
+
+    monkeypatch.setattr("app.html_render.render_html_to_image", _fake_render_html_to_image)
+
+    resp = client.get(f"/api/frames/1/widgets/{widget.id}/preview/weather")
+    assert resp.status_code == 200, resp.text
+    assert resp.headers["content-type"] == "image/png"
+    assert calls, "html_render.render_html_to_image was never called -- preview endpoint didn't honor render_style"
+
+
 def test_config_save_truncates_an_overlong_tasks_name(client, db_session):
     client.post("/setup", data={"username": "alice", "password": "hunter22"})
     widget = _add_tasks_widget(db_session)