Add a dayTintProvider plugin hook

Lets a plugin colour-code days in the Plan sidebar. trip-segments can
already announce "Kanazawa begins" in the day plan, but it can't make
day 12 look like it belongs to Kanazawa.

dayScheduleProvider can't cover this: it renders a fixed row, and it's
capped at 60 items per provider, so one item per day would tint the
first 60 days of a six-month trip and silently stop. This hook is
bounded by day count instead.

GET /api/day-tints/:tripId returns per-day tones for three regions of a
day card - the number badge (also the mobile day chip), the header row,
and the expanded activity list. `tone` is a shorthand filling every
region a contribution doesn't name; unnamed regions render as they do
today. Tones only, never hex, so the host can pick the alpha per theme
and per region.

One contribution per day, resolved whole: first entry wins within a
provider, first granted provider wins across plugins.

server/src/nest/plugins/protocol/envelope.ts

@@ -327,6 +327,7 @@ export const KNOWN_PERMISSIONS = [
   'hook:map-layer-provider',
   'hook:route-provider',
   'hook:day-schedule-provider',
+  'hook:day-tint-provider',
   'hook:pdf-section-provider',
   'hook:atlas-layer-provider',
   'hook:journal-entry-provider',