widgets: stop aborting on machines without backlight/battery/pulseaudio

The release profile builds with panic = "abort", so a panic in any helper
thread took the whole dashboard down:

- backlight: WalkDir yields /sys/class/backlight itself, so detect() was
  always true and new() picked the class dir as the device, then panicked
  reading brightness. Use min_depth(1) and tolerate read errors.
- battery: detect() only checked that UPower's DisplayDevice answered,
  which it does on every machine; use IsPresent. The monitor thread now
  returns an error instead of unwrapping, and publishes its first reading.
- pulseaudio: a failed connection panicked ("damn"); clicks before the
  first sink query unwrapped a None channel map. Both are now handled, and
  the connect loop no longer busy-spins.
- bar widget: don't underflow when narrower than the label column.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

src/widgets/pulseaudio.rs

@@ -48,10 +48,17 @@ fn monitor(
     context.borrow_mut().connect(None, FlagSet::NOFLAGS, None)?;
 
     loop {
-        mainloop.iterate(false);
+        match mainloop.iterate(true) {
+            IterateResult::Quit(_) | IterateResult::Err(_) => {
+                return Err("pulseaudio mainloop quit while connecting".into())
+            }
+            _ => (),
+        }
         match context.borrow().get_state() {
             State::Ready => break,
-            State::Failed | State::Terminated => panic!("damn"),
+            State::Failed | State::Terminated => {
+                return Err("could not connect to pulseaudio".into())
+            }
             _ => (),
         }
     }
@@ -120,15 +127,14 @@ fn monitor(
 
             let mut inner = local_inner.lock().unwrap();
             let mut introspector = local_context.borrow_mut().introspect();
-            match inner.last_click.take() {
-                Some(Change::Volume(volume)) => {
-                    // do something
-                    let mut vol = inner.channel_volume.unwrap();
+            // Clicks arriving before the first sink query finished have no
+            // channel map to work with; drop them instead of panicking.
+            match (inner.last_click.take(), inner.channel_volume) {
+                (Some(Change::Volume(volume)), Some(mut vol)) => {
                     vol.scale(Volume((volume * Volume::NORMAL.0 as f32).round() as u32));
                     introspector.set_sink_volume_by_name("@DEFAULT_SINK@", &vol, None);
                 }
-                Some(Change::VolumeInc(value)) => {
-                    let mut vol = inner.channel_volume.unwrap();
+                (Some(Change::VolumeInc(value)), Some(mut vol)) => {
                     // apply step to volumes
                     let step = (value * Volume::NORMAL.0 as f32).round() as i32;
                     if step > 0 {
@@ -141,10 +147,10 @@ fn monitor(
                     inner.channel_volume = Some(vol);
                     introspector.set_sink_volume_by_name("@DEFAULT_SINK@", &vol, None);
                 }
-                Some(Change::ToggleMute) => {
+                (Some(Change::ToggleMute), _) => {
                     introspector.set_sink_mute_by_name("@DEFAULT_SINK@", !inner.mute, None);
                 }
-                None => (),
+                _ => (),
             }
         }),
     );
@@ -165,7 +171,13 @@ fn monitor(
 fn start_monitor(events: Arc<Mutex<Events>>, inner: Arc<Mutex<InnerAudio>>, pipe: RawFd) {
     thread::Builder::new()
         .name("audiomon".to_string())
-        .spawn(move || monitor(events, inner, pipe).unwrap())
+        .spawn(move || {
+            // panic = "abort" in release: a failed audio connection must not
+            // kill the dashboard. The widget simply stays at 0.
+            if let Err(e) = monitor(events, inner, pipe) {
+                eprintln!("wldash: audio monitor stopped: {}", e);
+            }
+        })
         .unwrap();
 }