Manual per-calendar color choice for calendar mode

Each linked person can pin one of the panel's four non-black/white
colors (Yellow/Red/Blue/Green) to their own calendar instead of relying
on calendar_render.py's old auto-cycle-by-owner-name order -- owner-only,
like adding a calendar in the first place. Colors resolve against
whichever palette a frame actually renders with (including a custom
Advanced configuration override), so a pinned "Blue" stays this frame's
actual blue. Event color bars/dots are also bigger and rounded now
across agenda/week/month views, easier to tell apart at a glance.

server/app/calendar_feed.py

@@ -94,13 +94,17 @@ class CalendarSource:
     "caldav", url is the calendar's own URL, username/password its
     account credentials) -- see caldav_client.py. owner_display_name
     tags every event pulled from this source so a merged agenda can show
-    whose event is whose."""
+    whose event is whose. color_index (2-5, into
+    image_pipeline.DEFAULT_PALETTE_RGB) is this calendar's manually
+    pinned color, or None to fall back on calendar_render.py's old
+    auto-cycle-by-owner-name behavior -- see models.FrameCalendar."""
 
     owner_display_name: str
     kind: str
     url: str
     username: str = ""
     password: str = ""
+    color_index: int | None = None
 
 
 def merge_events(
@@ -127,6 +131,7 @@ def merge_events(
             continue
         for event in events:
             event["owner_display_name"] = source.owner_display_name
+            event["color_index"] = source.color_index
             merged.append(event)
 
     merged.sort(key=lambda e: e["start"])