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.

plugin-sdk/test/sdk.test.ts

@@ -86,8 +86,8 @@ describe('validateManifest', () => {
     expect(r.manifest?.permissions).toEqual(permissions);
   });
 
-  it('accepts hook:map-layer-provider, hook:day-schedule-provider and geolocation:read', () => {
-    const r = validateManifest({ ...base, permissions: ['hook:map-layer-provider', 'hook:day-schedule-provider', 'geolocation:read'] });
+  it('accepts hook:map-layer-provider, hook:day-schedule-provider, hook:day-tint-provider and geolocation:read', () => {
+    const r = validateManifest({ ...base, permissions: ['hook:map-layer-provider', 'hook:day-schedule-provider', 'hook:day-tint-provider', 'geolocation:read'] });
     expect(r.errors).toEqual([]);
     expect(r.ok).toBe(true);
   });