Feature: HTMX + Jinja2 Frontend ersetzt Next.js komplett

- Kein Node.js, kein npm, kein Build-Schritt mehr
- HTMX 2.0.4 + PicoCSS 2 vendored in backend/static/
- Jinja2 Templates für alle 9 Seiten (Dashboard, ZFS, Snapshots,
  Shares, Identities, Logs, Services, Navigator, Login)
- HTMX Fragments für Live-Updates (30s Polling Dashboard)
- JWT als httpOnly Cookie statt localStorage
- Disk Usage zeigt TB/PB korrekt (Jinja2 serverseitig formatiert)
- Update-safe: nur Python-Deps, keine npm-Abhängigkeiten

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/templates/fragments/disk_usage.html

@@ -0,0 +1,29 @@
+{% macro fmt_bytes(b) %}
+  {%- if b == 0 %}0 B
+  {%- elif b < 1048576 %}{{ "%.1f"|format(b/1024) }} KB
+  {%- elif b < 1073741824 %}{{ "%.1f"|format(b/1048576) }} MB
+  {%- elif b < 1099511627776 %}{{ "%.1f"|format(b/1073741824) }} GB
+  {%- else %}{{ "%.1f"|format(b/1099511627776) }} TB
+  {%- endif %}
+{%- endmacro %}
+
+{% if not filesystems %}
+<p>Keine Dateisysteme gefunden.</p>
+{% else %}
+<div style="display:grid;grid-template-columns:repeat(auto-fill,minmax(240px,1fr));gap:1rem">
+{% for fs in filesystems %}
+  <article style="margin:0;padding:1rem">
+    <strong>{{ fs.mountpoint }}</strong>
+    <div class="progress-bar-wrap" style="margin:0.4rem 0">
+      <div class="progress-bar {% if fs.capacity > 85 %}bar-red{% elif fs.capacity > 70 %}bar-yellow{% else %}bar-blue{% endif %}" style="width:{{ fs.capacity }}%"></div>
+    </div>
+    <div style="display:grid;grid-template-columns:1fr 1fr 1fr;font-size:0.8rem;gap:0.25rem">
+      <div><div style="color:var(--pico-muted-color)">Total</div><strong>{{ fmt_bytes(fs.total) }}</strong></div>
+      <div><div style="color:var(--pico-muted-color)">Belegt</div><strong>{{ fmt_bytes(fs.used) }}</strong></div>
+      <div><div style="color:var(--pico-muted-color)">Frei</div><strong>{{ fmt_bytes(fs.available) }}</strong></div>
+    </div>
+    <small style="color:var(--pico-muted-color)">{{ fs.filesystem }}</small>
+  </article>
+{% endfor %}
+</div>
+{% endif %}