Start face indexing from Settings

Beside the thumbnail sweep, because it is the same kind of thing: a job
that runs for an hour, is asked for once, and reports into the activity
list above it. It is also downstream of that sweep -- detection reads the
proxies it builds -- so the two belong in that order, and the coverage
line says how many images are waiting on a proxy rather than only how
many are left to index.

The button drives the Identity Manager's own state rather than a second
copy, so it cannot disagree with that screen about whether a pass is
running, and either place can start or stop it.

The pass now opens an activity row. The caption promises progress will
appear in the list above, and without a row it would not: the button
would be the only sign anything was happening, invisible from every
other screen.

Coverage is read when the Settings page opens. The figures live in the
catalog and this page deliberately holds no session, so they arrive
through a closure rather than being kept current -- they are only ever
looked at while the page is on screen, and the check is two counts and an
indexed scan.

Also adds DARKROOM_NO_SYNC. Redirecting XDG_DATA_HOME isolates a test
launch's catalog and thumbnails but not its server, and I found that out
by pushing a test catalog over the live one. The guard sits in
start_derived_sync rather than at its three call sites, because the sweep
firing a sync is correct and a flag checked in three places is one that
gets missed in a fourth.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

ui/dr-ui/src/settings_ui.rs

@@ -281,15 +281,26 @@ fn index_of<T: PartialEq>(all: &[T], value: &T) -> i32 {
 /// `on_budget_changed` runs when a cache ceiling moves, so the caller can
 /// enforce it against the catalog — this module has no connection and must not
 /// grow one.
-pub fn wire<F>(window: &AppWindow, controller: Rc<SettingsController>, on_budget_changed: F)
-where
+pub fn wire<F, G>(
+    window: &AppWindow,
+    controller: Rc<SettingsController>,
+    on_budget_changed: F,
+    on_open: G,
+) where
     F: Fn(&Settings) + 'static,
+    G: Fn(&AppWindow) + 'static,
 {
     let on_budget_changed = Rc::new(on_budget_changed);
+    // Run when the page opens, to fill in figures this module does not own —
+    // the face-indexing coverage, which lives in the catalog. Read here rather
+    // than kept up to date continuously, because it is only ever looked at
+    // when this page is on screen and it is two counts and an indexed scan.
+    let on_open: Rc<dyn Fn(&AppWindow)> = Rc::new(on_open);
 
     // --- opening and closing -------------------------------------------
     {
         let weak = window.as_weak();
+        let on_open = on_open.clone();
         let ctl = controller.clone();
         window.on_settings_open(move || {
             let Some(w) = weak.upgrade() else { return };
@@ -299,6 +310,7 @@ where
             // back over the newer one.
             *ctl.settings.borrow_mut() = ctl.store.load();
             render(&w, &ctl);
+            on_open(&w);
             w.set_show_settings(true);
         });
     }