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/Dockerfile

@@ -36,25 +36,62 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
     tzdata fontconfig fonts-dejavu-core nodejs npm \
     && rm -rf /var/lib/apt/lists/*
 
+# EXPERIMENTAL, likely unmergeable as-is -- see below. System libs a
+# headless Chromium needs (app/html_render.py, the weather widget's
+# opt-in "modern" render style), trimmed from Playwright's own full
+# `install-deps chromium` list to just what a headless (no Xvfb),
+# Latin-text-plus-emoji use case needs: dropped xvfb (only needed for a
+# *headed* browser) and the CJK/Cyrillic/Thai locale font packages
+# (fonts-ipafont-gothic, fonts-wqy-zenhei, fonts-tlwg-loma-otf,
+# xfonts-cyrillic, xfonts-scalable, fonts-freefont-ttf, fonts-unifont) --
+# fonts-noto-color-emoji is the one that actually matters here (real
+# color emoji in the weather icons, vs. WeasyPrint/Pango's monochrome
+# fallback glyphs in this feature's original spike).
+RUN apt-get update && apt-get install -y --no-install-recommends \
+    libasound2t64 libatk-bridge2.0-0t64 libatk1.0-0t64 libatspi2.0-0t64 \
+    libcairo2 libcups2t64 libdbus-1-3 libdrm2 libgbm1 libglib2.0-0t64 \
+    libnspr4 libnss3 libpango-1.0-0 libx11-6 libxcb1 libxcomposite1 \
+    libxdamage1 libxext6 libxfixes3 libxkbcommon0 libxrandr2 \
+    fonts-noto-color-emoji libfontconfig1 libfreetype6 fonts-liberation \
+    && rm -rf /var/lib/apt/lists/*
+
 COPY requirements.txt .
 # Split across several layers rather than one `pip install -r
 # requirements.txt` -- same Cloudflare single-blob/layer payload-size
 # limit as render-service's npm installs below. The single combined
 # layer was measured at ~113MB unpacked, over the limit on its own.
-# Isolating the three largest packages gets every layer's unpacked size
-# well clear of 100MB (sqlalchemy ~15MB, pillow ~19MB, pypdfium2 ~8MB,
-# the remaining `-r requirements.txt` layer ~71MB). Each package
-# version here still comes from requirements.txt (`pip install -r` for
-# everything that doesn't need its own layer skips these three, since
-# pip sees them already satisfied); the explicit versions below just
-# control *when* each installs -- same "single source of truth, just
-# splitting *when* it installs" tradeoff as the npm section's --no-save
-# comment below.
+# Isolating the largest packages gets every layer's unpacked size well
+# clear of 100MB (sqlalchemy ~15MB, pillow ~19MB, pypdfium2 ~8MB, the
+# remaining `-r requirements.txt` layer ~71MB). Each package version here
+# still comes from requirements.txt (`pip install -r` for everything that
+# doesn't need its own layer skips these, since pip sees them already
+# satisfied); the explicit versions below just control *when* each
+# installs -- same "single source of truth, just splitting *when* it
+# installs" tradeoff as the npm section's --no-save comment below.
 RUN pip install --no-cache-dir sqlalchemy==2.0.51
 RUN pip install --no-cache-dir pillow==12.3.0
 RUN pip install --no-cache-dir pypdfium2==5.12.1
+RUN pip install --no-cache-dir playwright==1.61.0
 RUN pip install --no-cache-dir -r requirements.txt
 
+# KNOWN LIKELY BLOCKER, not resolved by pulling this into its own layer:
+# `playwright install chromium-headless-shell` unpacks to ~262MB, and its
+# single `chrome-headless-shell` binary alone (measured: 181MB) is one
+# file -- unlike the pip/npm splits above (independently-installable
+# smaller packages moved into their own layers), a single 181MB file
+# can't be divided across multiple <100MB Docker layers by any ordinary
+# COPY/RUN restructuring; the whole file lands in whichever layer's diff
+# contains it. This almost certainly exceeds the same Cloudflare single-
+# blob/layer limit that forced the pip/npm splits elsewhere in this file
+# (see their comments) -- an actual push to this project's registry
+# hasn't been attempted (would require pushing to `main`, which triggers
+# deploy) to confirm, but there is no reason to expect a single 181MB
+# blob to fit where combined ~113MB of many small wheels didn't. Needs a
+# real resolution (a registry without this limit, hosting the browser
+# binary outside the image, etc.) before this branch can actually ship --
+# tracked as open, not silently assumed away.
+RUN playwright install chromium-headless-shell
+
 # render-service/'s dependencies installed as several separate layers
 # rather than one `npm install` covering all of them -- a from-scratch
 # push of this image once hit Cloudflare's payload-size limit on a