feat(tier2): event branding + UX polish — Block D

Backend
- Migration 0010 adds event_branding (one row per event; all fields
  nullable so a brand-new event renders with defaults)
- BrandingRepo with COALESCE/NULLIF upsert semantics: nil pointer
  preserves the existing value, "" clears the field to NULL
- internal/uploads package: ImageStore interface + LocalFSStore (dev),
  pure-stdlib decode + re-encode that strips EXIF and rejects anything
  that isn't valid JPEG/PNG. Size cap 2 MB, random 16-byte filenames
- GET /events/{id}/branding (viewer+) returns the row plus the
  AllowedFonts list so the frontend picker stays in sync
- PUT /events/{id}/branding (editor+) validates hex colours, font
  allowlist, and refuses image URLs whose path doesn't start with
  /uploads/ (blocks arbitrary-origin <img> smuggling on guest pages)
- POST /uploads/image (authed) → fresh CDN URL; GET /uploads/{file}
  serves with year-long cache (immutable random names)
- GET /access/{token} now embeds the host's branding so the RSVP page
  can render in their colours/font with their logo + cover
- docker-compose mounts a named volume for uploads
- Custom-domain sub-block deferred to Tier 3 per the plan

Frontend
- BrandingCard.vue: colour pickers, font dropdown, logo + cover upload
  with progressive disclosure, live preview pane that re-renders on
  every keystroke
- RSVP page applies branding via CSS vars at the section root, so
  primary colour theme + font cascade through every child card. Cover
  image renders as a banner above the form; logo lands in the header
- Submit button background switches to var(--brand-primary) when set
- Mounted on the event detail page below the guests block

Plus the small UX fixes from the e2e walkthrough:
- Nav: dropped the top-level "Events" link; the logo doubles as the
  home affordance (→ /dashboard when signed in, → / otherwise). Account
  + Billing + Sign out live under a profile dropdown (avatar with
  initials, opens on click, closes on outside-click / Esc / route nav)
- Renamed "Back to dashboard" → "Back to events" across event detail,
  billing, account, and new-event pages

Tests
- TestBrandingGetReturnsDefaults / TestBrandingPutPersists /
  TestBrandingPutRejectsBadInputs / TestUploadAndServeImage /
  TestUploadRejectsNonImage — all pass
- Domain tests for IsValidHexColor + IsAllowedFont
- Full integration suite green (176s)

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/pages/dashboard/events/[id].vue

@@ -728,7 +728,7 @@ function checkLabel(band?: string): string {
   <section v-else class="space-y-8">
     <div>
       <NuxtLink to="/dashboard" class="mb-2 inline-block text-sm text-zinc-400 hover:text-zinc-200">
-        ← Back to dashboard
+        ← Back to events
       </NuxtLink>
       <div class="flex items-center justify-between">
         <h1 class="text-2xl font-semibold">{{ event.name }}</h1>
@@ -1120,6 +1120,18 @@ function checkLabel(band?: string): string {
       </aside>
     </div>
 
+    <!-- Branding (Tier 2 Block D). Editor+ can change colours/logo/cover;
+         viewers see read-only. -->
+    <div v-if="event" class="mt-6">
+      <BrandingCard
+        :event-id="eventId"
+        :your-role="event.your_role"
+        :event-name="event.name"
+        :event-venue="event.venue"
+        :event-date="event.event_date"
+      />
+    </div>
+
     <!-- Analytics (Tier 2 Block E). Read-only summary; viewer+ can see it. -->
     <div v-if="event" class="mt-6">
       <AnalyticsCard :event-id="eventId" />