Design-System Sprint A: utilities.css + 948 Inline-Styles → Utility-Klassen, SW by-v1102

PHASE 1 — Sofort-Cleanup ohne Risiko:
- Neue Datei utilities.css mit ~25 Klassen für häufige Kombinationen:
  * text-xs-muted, text-xs-secondary, text-sm-muted, text-sm-secondary
  * flex-gap-2/3, flex-col-gap-2/3/4, flex-center-gap-1/2/3
  * flex-between, flex-1-min, mb-1/3, mt-1/3
  * icon-xs/sm/md/lg, label-block, caption
- index.html bindet utilities.css ein
- mb-3/mt-3 ergänzt (waren in design-system.css unvollständig)

PHASE 2 — .by-tab Modifier für Vereinheitlichung:
- .by-tabs.grid (mit --tab-cols Variable für Admin/Health/etc.)
- .by-tabs.sticky (Desktop vertikale Tabs für Admin)
- .by-tabs.wrap (Zuchthunde, flex-wrap statt scroll)
- .by-tabs.separated (Sitting, mit eigenem Hintergrund + Border)

PHASE 3 — Inline-Style → Klassen-Migration (Python-Script):
- 948 Inline-Styles entfernt (5101 → 4153, -18%)
- 962 Migrationen über 47 Page-Dateien
- Top-Treffer: admin.js (180), health.js (67), dog-profile.js (67),
  litters.js (62), settings.js (61), zuchthunde.js (51)
- Patterns: text-muted, text-secondary, text-danger, text-xs-muted,
  text-sm-muted, grid-2 (Duplikat-Bug behoben!), flex-col-gap-3,
  p-3/4, mb-2/3/4, hidden, w-full, flex-1, ...
- Bewahrt bestehende class-Attribute (mergt korrekt)

Alle 19 Tests grün. Kein visueller Diff erwartet (gleiche Property-Werte).

backend/static/js/pages/chat.js

@@ -178,7 +178,7 @@ window.Page_chat = (() => {
           </button>`}
           <div style="position:relative;flex-shrink:0">
             <div class="chat-conv-avatar" id="chat-partner-av" style="width:32px;height:32px;font-size:var(--text-sm)">?</div>
-            <span class="online-dot chat-avatar-dot" id="chat-partner-dot" style="display:none"></span>
+            <span class="online-dot chat-avatar-dot" id="chat-partner-dot" class="hidden"></span>
           </div>
           <span class="chat-thread-partner" id="chat-partner-name">…</span>
         </div>
@@ -188,7 +188,7 @@ window.Page_chat = (() => {
           </div>
         </div>
         <div class="chat-input-bar">
-          <input type="file" id="chat-photo-input" accept="image/*" style="display:none"
+          <input type="file" id="chat-photo-input" accept="image/*" class="hidden"
                  onchange="Page_chat._onPhotoSelected(this)">
           <button class="chat-photo-btn" onclick="document.getElementById('chat-photo-input').click()" title="Foto senden">
             <svg class="ph-icon"><use href="/icons/phosphor.svg#camera"></use></svg>