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/app/main.py

@@ -17,6 +17,7 @@ from __future__ import annotations
 
 import logging
 import time
+from contextlib import asynccontextmanager
 
 from fastapi import FastAPI, Request
 from fastapi.responses import FileResponse, HTMLResponse, RedirectResponse
@@ -24,7 +25,7 @@ from fastapi.staticfiles import StaticFiles
 from fastapi.templating import Jinja2Templates
 from sqlalchemy import select
 
-from . import logging_setup, migration
+from . import html_render, logging_setup, migration
 from .auth import (
     browser_token_valid,
     current_user,
@@ -46,7 +47,20 @@ logging_setup.configure_logging()
 # Schema + legacy-config import, before the first request is served.
 migration.run_migrations()
 
-app = FastAPI(title="ESPresso Frame Server")
+@asynccontextmanager
+async def _lifespan(app: FastAPI):
+    """Startup does nothing browser-related -- html_render.start() is
+    lazy (only the weather widget's opt-in "modern" render style ever
+    triggers it, see that module's docstring), so a deployment that
+    never uses it never launches Chromium or needs Playwright's browser
+    binaries installed. Shutdown calls html_render.stop() unconditionally
+    (a no-op if it was never started) so a server restart never leaves
+    an orphaned Chromium process running."""
+    yield
+    html_render.stop()
+
+
+app = FastAPI(title="ESPresso Frame Server", lifespan=_lifespan)
 templates = Jinja2Templates(directory="app/templates")