feat: Extensions settings tab + capture dialog redesign with Advanced options

Settings/Extensions tab (admin-only):
- New 'Extensions' tab between Cookies and Storage
- ExtensionsTab component: shows uBlock Origin Lite card with pill toggle
- Reads ublock_enabled from instance settings; patch via existing PATCH endpoint
- Shows ublock_ext_available status from server (whether ARCHIVR_UBLOCK_EXT is set)

Instance settings:
- Add ublock_enabled BOOLEAN (default true) to instance_settings auth DB table
- Idempotent ALTER TABLE migration in initialize_auth_schema()
- get/update_instance_settings include ublock_enabled
- GET /api/admin/instance-settings now also returns ublock_ext_available (computed
  from ARCHIVR_UBLOCK_EXT env var at request time)
- PATCH /api/admin/instance-settings accepts ublock_enabled

Per-capture override:
- CaptureBody gains ublock_enabled: Option<bool>
- CaptureConfig gains ublock_enabled: Option<bool>
- singlefile::save() gains ublock_enabled_override: Option<bool> param
- Capture handler resolves: body override > global instance setting > env var
- submitCapture(aid, loc, qual, extensions) in api.js passes ublock_enabled

Capture dialog redesign:
- Archive button: full-width, 13px padding, min-width 220px, primary CTA
- Cancel: full-width but text-style, below Archive
- ‹Advanced options› chevron toggle (rotates on open)
- Expanded panel shows uBlock toggle for this capture session
- Loads global ublock_enabled default from instance settings on mount

Styles:
- .ext-toggle pill switch (44×24 and 36×20 small variant)
- .ext-card for Settings Extensions tab
- .capture-advanced + .capture-advanced-panel + .capture-chevron
- .capture-ext-row / .capture-ext-label / .capture-ext-name / .capture-ext-desc
- .form-hint utility class

frontend/src/api.js

@@ -95,9 +95,13 @@ export async function fetchTags(archiveId) {
   return getJson(`/api/archives/${archiveId}/tags`);
 }
 
-export async function submitCapture(archiveId, locator, quality = null) {
+export async function submitCapture(archiveId, locator, quality = null, extensions = null) {
   const payload = { locator }
   if (quality && quality !== 'best') payload.quality = quality
+  // extensions: { ublock_enabled?: bool } — per-capture overrides
+  if (extensions && typeof extensions.ublock_enabled === 'boolean') {
+    payload.ublock_enabled = extensions.ublock_enabled
+  }
   const res = await fetch(`/api/archives/${archiveId}/captures`, {
     method: "POST",
     headers: { "Content-Type": "application/json" },