fix: refactor breadcrumb partial to WAI-ARIA best-practice pattern

- Replace <div>+<i> structure with <nav aria-label>+<ol>+<li>
- Move separators from DOM to CSS pseudo-elements (li+li::before)
- Add hover/focus-visible states and color differentiation
- Replace hard-coded px values with design tokens
- Explicitly reset shell nav/ol/li rules for self-contained styling
- Add i18n key for breadcrumb nav landmark label

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

i18n/default_de.json

@@ -1316,5 +1316,6 @@
   "Fail": "Fehler",
   "Environment": "Umgebung",
   "Modules": "Module",
-  "Version": "Version"
+  "Version": "Version",
+  "Breadcrumb": "Breadcrumb"
 }