feat(design): v2 becomes the default — drop beta conversation

Atmospheric design is now the default for everyone:
- User.getDesignVersion() returns 'v2' when unset (was 'v1')
- All Twig templates default the cookie read to 'v2'
- SettingsView 'Design (beta)' section removed entirely along with the
  selectDesign() handler and currentDesign computed

The /api/user/design endpoint stays in place so the design_version column
can still be set programmatically (or migrated later), but the UI no
longer exposes it as a user-facing choice. Existing users with explicit
'v1' in their DB row continue to see v1 — their preference persists.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/views/SettingsView.vue

@@ -38,36 +38,6 @@
       </div>
     </section>
 
-    <section class="settings__section">
-      <h2 class="settings__section-title">Design (beta)</h2>
-      <p class="settings__hint">
-        Try the new atmospheric look. Your theme picks above still apply —
-        v2 just renders them as dusks over a Camogli harbor backdrop.
-      </p>
-      <div class="design-toggle" role="radiogroup" aria-label="Design version">
-        <button
-          type="button"
-          role="radio"
-          :aria-checked="currentDesign === 'v1'"
-          :class="['design-toggle__opt', { 'design-toggle__opt--active': currentDesign === 'v1' }]"
-          @click="selectDesign('v1')"
-        >
-          <span class="design-toggle__label">Original</span>
-          <span class="design-toggle__sub">cream &amp; terracotta</span>
-        </button>
-        <button
-          type="button"
-          role="radio"
-          :aria-checked="currentDesign === 'v2'"
-          :class="['design-toggle__opt', { 'design-toggle__opt--active': currentDesign === 'v2' }]"
-          @click="selectDesign('v2')"
-        >
-          <span class="design-toggle__label">Atmospheric</span>
-          <span class="design-toggle__sub">harbor dusks · beta</span>
-        </button>
-      </div>
-    </section>
-
     <section v-if="!isStandalone" class="settings__section">
       <h2 class="settings__section-title">Install app</h2>
       <p class="settings__hint">
@@ -207,34 +177,12 @@ const { saveTheme } = useTheme()
 const { isStandalone, isIOS, canPromptInstall, install } = usePwaInstall()
 
 const currentTheme = computed(() => auth.user?.theme ?? 'warm-craft')
-const currentDesign = computed(() => auth.user?.designVersion ?? 'v1')
 const showIosInstructions = ref(false)
 
 function select(themeId: string) {
   saveTheme(themeId)
 }
 
-async function selectDesign(version: 'v1' | 'v2') {
-  if (currentDesign.value === version) return
-  const prev = currentDesign.value
-  // Optimistic local update so the SPA flips immediately.
-  if (auth.user) auth.user.designVersion = version
-  document.documentElement.setAttribute('data-design', version)
-  try {
-    const res = await fetch('/api/user/design', {
-      method:  'PATCH',
-      headers: { 'Content-Type': 'application/json' },
-      body:    JSON.stringify({ designVersion: version }),
-    })
-    if (!res.ok) throw new Error('save failed')
-    // Cookie is set in the response — read-once nothing else to do here.
-  } catch {
-    // Revert on failure.
-    if (auth.user) auth.user.designVersion = prev
-    document.documentElement.setAttribute('data-design', prev)
-  }
-}
-
 async function onNativeInstall() {
   const accepted = await install()
   // If the native prompt failed for any reason (rare — e.g. the event