Actually render emoji in calendar event titles; stack duplicate shared events

The earlier fix stripped emoji instead of rendering them, which wasn't
what was asked for. Event titles now draw with two fonts: the usual
default font for text, and a vendored monochrome emoji font (Noto
Emoji, OFL-1.1) for actual emoji runs, so they show up as real glyphs
instead of a tofu box or nothing at all. Monochrome rather than color,
since reliably rendering COLR/CBDT color glyphs depends on how Pillow's
FreeType was built -- not something to depend on across deployments.

Also: events sharing the exact same title and time across different
calendars (e.g. a shared family event synced onto more than one
person's calendar) now collapse into one row instead of showing twice,
with a color bar split between every contributing calendar so it's
still clear whose event it is.

server/README.md

@@ -224,6 +224,20 @@ Pages: `/` (routing hub), `/setup`, `/login`, `/claim`, `/settings`,
   explicit, informed call by the project owner, not a default -- anyone
   redistributing this project (vs. just self-hosting it) should
   re-evaluate that tradeoff for their own situation before doing so.
+- Calendar event titles can contain emoji, which `ImageFont.load_default()`
+  (used for every other bit of text this project renders) has no glyphs
+  for -- PIL/FreeType substitute a visible ".notdef" tofu box rather than
+  skipping the codepoint. `app/calendar_render.py` draws emoji runs with
+  a vendored monochrome font instead (`app/fonts/NotoEmoji.ttf`, Google's
+  Noto Emoji, OFL-1.1 -- license text alongside it at
+  `app/fonts/NotoEmoji-OFL.txt`), the one deliberate exception to this
+  project's usual "no new font/icon assets" default elsewhere in
+  calendar_render.py -- there's no way to hand-draw arbitrary emoji with
+  primitives the way the weather icons are. Monochrome rather than a
+  color emoji font on purpose: reliably rendering COLR/CBDT color glyphs
+  depends on how Pillow's FreeType was built, which isn't something this
+  project controls in every deployment environment, and solid black
+  reads cleanly against everything else this module draws in black.
 - The 6-color palette RGB values in `app/image_pipeline.py`
   (`DEFAULT_PALETTE_RGB`) are approximations, not measured values
   (Waveshare doesn't publish exact color primaries for this panel).