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-server/src/routes.rs

@@ -653,6 +653,8 @@ struct CaptureBody {
     /// Optional quality cap for yt-dlp sources: `"best"` or any `"NNNp"` string
     /// (e.g. `"1080p"`, `"720p"`, `"2160p"`). Absent or `"best"` → highest available.
     quality: Option<String>,
+    /// Per-capture uBlock override.  Absent → use global instance setting.
+    ublock_enabled: Option<bool>,
 }
 
 #[derive(Debug, serde::Deserialize)]
@@ -733,14 +735,26 @@ async fn capture_handler(
     let job_uid = database::create_capture_job(&conn, &archive_id)?;
     drop(conn);
 
-    // Load cookie rules from the auth DB to pass into the capture background task.
-    let cookie_rules = {
+    // Load cookie rules and global uBlock setting from the auth DB.
+    let (cookie_rules, global_ublock) = {
         match database::open_auth_db(&state.auth_db_path) {
-            Ok(conn) => database::list_cookie_rules(&conn).unwrap_or_default(),
-            Err(_) => vec![],
+            Ok(conn) => {
+                let rules = database::list_cookie_rules(&conn).unwrap_or_default();
+                let ublock = database::get_instance_settings(&conn)
+                    .map(|s| s.ublock_enabled)
+                    .unwrap_or(true);
+                (rules, ublock)
+            }
+            Err(_) => (vec![], true),
         }
     };
-    let capture_config = capture::CaptureConfig { cookie_rules };
+    // Per-capture body overrides global; if body doesn't specify, use the global setting.
+    // The resolved bool is then passed as Some(_) to singlefile, overriding the env var.
+    let effective_ublock = body.ublock_enabled.unwrap_or(global_ublock);
+    let capture_config = capture::CaptureConfig {
+        cookie_rules,
+        ublock_enabled: Some(effective_ublock),
+    };
 
     // Spawn background capture.
     let locator = body.locator.trim().to_string();
@@ -1020,10 +1034,20 @@ async fn patch_me(
 async fn get_instance_settings_handler(
     State(state): State<AppState>,
     auth_user: AuthUser,
-) -> Result<Json<database::InstanceSettings>, ApiError> {
+) -> Result<Json<serde_json::Value>, ApiError> {
     auth_user.require_role(ROLE_ADMIN)?;
     let conn = database::open_auth_db(&state.auth_db_path)?;
-    Ok(Json(database::get_instance_settings(&conn)?))
+    let settings = database::get_instance_settings(&conn)?;
+    let ublock_ext_available = std::env::var("ARCHIVR_UBLOCK_EXT")
+        .ok()
+        .filter(|s| !s.is_empty())
+        .map(|p| std::path::Path::new(&p).is_dir())
+        .unwrap_or(false);
+    let mut val = serde_json::to_value(&settings).unwrap_or_default();
+    if let Some(obj) = val.as_object_mut() {
+        obj.insert("ublock_ext_available".into(), serde_json::Value::Bool(ublock_ext_available));
+    }
+    Ok(Json(val))
 }
 
 async fn update_instance_settings_handler(
@@ -1038,6 +1062,7 @@ async fn update_instance_settings_handler(
     if let Some(v) = body.public_entry_content_enabled { settings.public_entry_content_enabled = v; }
     if let Some(v) = body.open_registration_enabled { settings.open_registration_enabled = v; }
     if let Some(v) = body.default_entry_visibility { settings.default_entry_visibility = v; }
+    if let Some(v) = body.ublock_enabled { settings.ublock_enabled = v; }
     database::update_instance_settings(&conn, &settings)?;
     Ok(StatusCode::NO_CONTENT)
 }
@@ -1367,6 +1392,7 @@ struct UpdateInstanceSettingsBody {
     public_entry_content_enabled: Option<bool>,
     open_registration_enabled: Option<bool>,
     default_entry_visibility: Option<u32>,
+    ublock_enabled: Option<bool>,
 }
 
 async fn admin_list_users(