Fix stuck search dropdown; add plugin-tab nested-settings filter

Global search:
- Close the results dropdown on input blur (guarded, with a short delay)
  so it reliably dismisses when focus leaves — previously it could linger
  because the only outside-close was a document click that Alpine/HTMX
  handlers can swallow.
- Clear the query text after navigating to a result so refocusing the box
  doesn't re-open stale results.
- Also dismiss on htmx:afterSwap (tab changes / navigation).

Per-tab filter (now on plugin tabs too):
- Render the shared settings_filter box in the plugin Configuration panel.
  It auto-wires: the delegated input handler and filterScope already target
  .plugin-config-tab.
- Teach applyTabFilter to reveal matches inside collapsed nested sections
  (render_nested_section defaults them shut), hide nested-section wrappers
  with no matches, and restore the original collapsed layout when cleared
  (only re-collapsing sections the filter itself opened).
- Count a visible nested-section as content for its parent heading so the
  heading isn't hidden while a subsection below still has matches.

Adds a plugin-config render test (filter box + nested anchors + tooltips).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014gZxznuxw8L92FUMBN3Nqz

web_interface/templates/v3/partials/plugin_config.html

@@ -999,6 +999,7 @@
             {# Configuration Form Panel #}
             <div class="bg-gray-50 rounded-lg p-4">
                 <h3 class="text-md font-medium text-gray-900 mb-3">Configuration</h3>
+                {{ ui.settings_filter("Filter this plugin's settings…") }}
                 <div class="space-y-4 max-h-96 overflow-y-auto pr-2">
                     {% if schema and schema.properties %}
                         {# Use property order if defined, otherwise use natural order #}