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

crates/archivr-core/src/capture.rs

@@ -77,6 +77,11 @@ impl PlatformMetadata {
 #[derive(Debug, Clone, Default)]
 pub struct CaptureConfig {
     pub cookie_rules: Vec<database::CookieRule>,
+    /// Override for uBlock Origin Lite during WebPage captures.
+    /// `None` = derive from `ARCHIVR_UBLOCK` env var (existing behaviour).
+    /// `Some(false)` = disabled by admin/user choice for this capture.
+    /// `Some(true)` = explicitly enabled (still needs `ARCHIVR_UBLOCK_EXT` to be valid).
+    pub ublock_enabled: Option<bool>,
 }
 
 /// Resolves which cookies apply to `url` by evaluating all rules in ordinal order.
@@ -1010,7 +1015,7 @@ pub fn perform_capture(
 
     // Source: web page — archive as a self-contained HTML snapshot via single-file-cli
     if source == Source::WebPage {
-        match downloader::singlefile::save(locator, store_path, &timestamp, &cookies) {
+        match downloader::singlefile::save(locator, store_path, &timestamp, &cookies, config.ublock_enabled) {
             Ok(result) => {
                 let file_extension = ".html".to_string();
                 let temp_html = store_path