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/backlight.rs

@@ -32,23 +32,33 @@ pub struct Backlight {
     dirty: bool,
 }
 
+/// First device under /sys/class/backlight, if any. `min_depth(1)` skips the
+/// class directory itself, which otherwise got picked up as a "device".
+fn find_device() -> Option<PathBuf> {
+    WalkDir::new("/sys/class/backlight")
+        .min_depth(1)
+        .max_depth(1)
+        .into_iter()
+        .flatten()
+        .map(|e| e.path().to_path_buf())
+        .next()
+}
+
 impl Backlight {
     fn update(&mut self) {
-        self.cur = read_file_as_u64(self.device_path.join("brightness").as_path()).unwrap();
-        self.max = read_file_as_u64(self.device_path.join("max_brightness").as_path()).unwrap();
+        if let Ok(cur) = read_file_as_u64(self.device_path.join("brightness").as_path()) {
+            self.cur = cur;
+        }
+        if let Ok(max) = read_file_as_u64(self.device_path.join("max_brightness").as_path()) {
+            self.max = max;
+        }
     }
 
     pub fn new(path: Option<&str>, fm: &mut FontMap, font: &'static str, size: f32) -> BarWidget {
-        let mut device_path = None;
-        if let Some(path) = path {
-            device_path = Some(Path::new("/sys/class/backlight").to_path_buf().join(path));
-        } else {
-            for entry in WalkDir::new("/sys/class/backlight") {
-                if let Ok(entry) = entry {
-                    device_path = Some(entry.path().to_path_buf());
-                }
-            }
-        }
+        let device_path = match path {
+            Some(path) => Some(Path::new("/sys/class/backlight").join(path)),
+            None => find_device(),
+        };
         let mut dev = Backlight {
             device_path: device_path.expect("did not find backlight device"),
             cur: 0,
@@ -61,18 +71,15 @@ impl Backlight {
     }
 
     pub fn detect() -> bool {
-        for entry in WalkDir::new("/sys/class/backlight") {
-            if entry.is_ok() {
-                return true;
-            }
-        }
-        return false;
+        find_device().is_some()
     }
 
     pub fn set(&mut self, brightness: f32) {
-        let val = (self.max as f32 * brightness) as u64;
+        let val = (self.max as f32 * brightness.clamp(0.0, 1.0)) as u64;
         let _ = write_file_as_u64(self.device_path.join("brightness").as_path(), val);
-        self.cur = read_file_as_u64(self.device_path.join("brightness").as_path()).unwrap();
+        if let Ok(cur) = read_file_as_u64(self.device_path.join("brightness").as_path()) {
+            self.cur = cur;
+        }
     }
 }
 
@@ -84,7 +91,7 @@ impl BarWidgetImpl for Backlight {
         self.dirty
     }
     fn value(&mut self) -> f32 {
-        if self.cur > self.max {
+        if self.max == 0 || self.cur > self.max {
             // what.
             return 1.0;
         }