feat(ui): Disk, Traefik, UniFi and Settings say things plainly (#5212)

#5212 "Plain labels: no raw ids, scopes, metric keys or unexplained jargon",
batch 3:

- Disk page: each tile says what it holds. "Writable layers" becomes
  "Container files" and Build cache says it is safe to clear. Shared is
  "Shared with other images" with a tooltip, and "dangling" is gone.
  Untagged images show by id instead of "<none>", a capped list says
  "The 50 largest of N images", and each host shows "as of" its report.
- Traefik: an expired certificate reads "expired 12 days ago", not "-12
  days" (cert_left). The Traffic empty state knows whether the access log
  is on: off points to Settings → Plugins → Traefik; on says to check the
  mounted log path.
- Moments use |ago with an exact UTC tooltip: UniFi last seen, alarms,
  events, speed test and DPI; AI token last used.
- Settings: the Auth tab is "Authentication" like its breadcrumb, plugin
  fields read "Metrics URL" not metrics_url (field_label), a source's pull
  interval is "every 1h", and the last report time drops the ISO "T".

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

steward/templates/settings/plugin_detail.html

@@ -72,14 +72,14 @@
                       background:var(--bg);border-radius:5px;border:1px solid var(--border);
                       font-size:0.82rem;color:var(--text-muted);">
             <span style="flex-shrink:0;">📋</span>
-            <span><strong style="color:var(--text);">{{ cfg_key }}</strong> — list config, edit in
+            <span><strong style="color:var(--text);">{{ field_label(cfg_key) }}</strong> — list config, edit in
               <code>plugin.yaml</code> ({{ cfg_default | length }} item{{ 's' if cfg_default | length != 1 }}).</span>
           </div>
 
         {% elif cfg_default is mapping %}
           <div style="padding:0.75rem;background:var(--bg);border-radius:5px;border:1px solid var(--border);">
             <div style="font-size:0.74rem;color:var(--text-muted);text-transform:uppercase;
-                        letter-spacing:0.05em;margin-bottom:0.6rem;">{{ cfg_key }}</div>
+                        letter-spacing:0.05em;margin-bottom:0.6rem;">{{ field_label(cfg_key) }}</div>
             {% set sub_cfg = plugin._config.get(cfg_key, cfg_default) if plugin._config.get(cfg_key, cfg_default) is mapping else cfg_default %}
             <div style="display:grid;gap:0.5rem;">
               {% for sub_key, sub_default in cfg_default.items() %}
@@ -90,11 +90,11 @@
                        id="plugin-{{ plugin._dir }}-{{ cfg_key }}-{{ sub_key }}"
                        {% if sub_cfg.get(sub_key, sub_default) %}checked{% endif %}>
                 <label for="plugin-{{ plugin._dir }}-{{ cfg_key }}-{{ sub_key }}"
-                       style="margin:0;font-size:0.85rem;color:var(--text);">{{ sub_key }}</label>
+                       style="margin:0;font-size:0.85rem;color:var(--text);">{{ field_label(sub_key) }}</label>
               </div>
               {% else %}
               <div class="form-group" style="margin:0;">
-                <label style="font-size:0.82rem;">{{ sub_key }}</label>
+                <label style="font-size:0.82rem;">{{ field_label(sub_key) }}</label>
                 <input type="text"
                        name="plugin.{{ plugin._dir }}.{{ cfg_key }}.{{ sub_key }}"
                        value="{{ sub_cfg.get(sub_key, sub_default) }}">
@@ -111,12 +111,12 @@
                    id="plugin-{{ plugin._dir }}-{{ cfg_key }}"
                    {% if plugin._config.get(cfg_key, cfg_default) %}checked{% endif %}>
             <label for="plugin-{{ plugin._dir }}-{{ cfg_key }}"
-                   style="margin:0;font-size:0.85rem;color:var(--text);">{{ cfg_key }}</label>
+                   style="margin:0;font-size:0.85rem;color:var(--text);">{{ field_label(cfg_key) }}</label>
           </div>
 
         {% else %}
           <div class="form-group" style="margin:0;">
-            <label style="font-size:0.82rem;">{{ cfg_key }}</label>
+            <label style="font-size:0.82rem;">{{ field_label(cfg_key) }}</label>
             <input type="{% if 'password' in cfg_key or 'secret' in cfg_key %}password{% else %}text{% endif %}"
                    name="plugin.{{ plugin._dir }}.{{ cfg_key }}"
                    value="{% if 'password' in cfg_key or 'secret' in cfg_key %}{% else %}{{ plugin._config.get(cfg_key, cfg_default) }}{% endif %}"