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/static/frame_config.js

@@ -260,6 +260,24 @@ document.getElementById('photo-palette-reset').addEventListener('click', () => {
   savePalette('photo-palette', 'photo_palette', { photo_palette_reset: 'true', photo_dither_strength: '1' });
 });
 
+// ---- Theme ----
+
+document.getElementById('theme-save').addEventListener('click', async () => {
+  const body = new URLSearchParams({ theme: document.getElementById('theme-select').value });
+  try {
+    const resp = await fetch(`${window.FRAME_API}/config`, {
+      method: 'POST',
+      headers: { 'Content-Type': 'application/x-www-form-urlencoded' },
+      body,
+    });
+    if (!resp.ok) throw new Error(await apiError(resp));
+    showStatus(true, 'Saved.');
+    loadPreview();
+  } catch (e) {
+    showStatus(false, e.message);
+  }
+});
+
 // ---- Preview: current photo vs. how it renders with saved settings ----
 // Scoped to this frame's photo widget (window.PHOTO_WIDGET_PREVIEW_API,
 // set by the template) rather than window.FRAME_API -- palette/color/