v0.315.0: style the app page's tab bar, and reuse the hero classes that already have rules

FOUND IN A BROWSER, and findable no other way. Every endpoint-level marker passed and the page was
unusable: the tab bar rendered as four bare underlined links run together, and the app logo filled a
third of the screen. The shell had been written with new class names — app-hero, app-hero-logo,
app-tabs — and only the first two of those had any CSS at all, under the names the design system
already uses.

  - the shell now reuses .app-info-hero / .app-info-logo / .app-info-hero-text, which carry the
    80px logo box, the hairline border and the 2px radius. Inventing a parallel name is what left
    the logo unsized; REUSE.md's first rule, learned again.
  - .app-tabs / .app-tab / .app-tab-active are genuinely new and are now styled to design system
    v2: 2px radius, hairline --line border, no shadow, --text-2 for an inactive tab and
    --blue-bright only for the one the household is on (blue marks where you are, not a deviation),
    the active tab sitting on the bar's own line, and a narrower gutter under 640px.
  - .after-start-card gets a blue left hairline — it is a request, not a fault, so no amber.
  - .app-faq gets its dt/dd rules.

TestAppPage_DeployedDefaultsToSettings sliced the header at `class="app-hero"`; it now slices at the
reused class and fails loudly if either marker is gone, instead of slicing on -1. The 36 app-page
parity fixtures are recaptured — and no other fixture moved, which is the evidence that the shell
change reaches only the app page.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0117kRTSgs3rJWUFGxyoWN6Y

controller/internal/web/testdata/i18n_parity/app_guide_available.html

@@ -182,11 +182,11 @@
     </div>
 </div>
 
-<div class="app-hero">
-    <img class="app-hero-logo" src="/static/assets/privatebin-logo.svg"
+<div class="app-info-hero">
+    <img class="app-info-logo" src="/static/assets/privatebin-logo.svg"
          alt="PrivateBin" data-fallback="/static/app-placeholder.svg"
          onerror="if(!this.dataset.step){this.dataset.step='1';this.src='\/static\/assets\/privatebin-logo.png';}else if(this.dataset.fallback&&this.dataset.step==='1'){this.dataset.step='2';this.src=this.dataset.fallback;}else{this.onerror=null;this.style.visibility='hidden';}">
-    <div class="app-hero-text">
+    <div class="app-info-hero-text">
         <p class="app-info-tagline">Titkosított jegyzetek.</p>
         <div class="stack-meta-badges">
             <span class="meta-badge">~64M RAM</span>