fix(player): give the seek bar a thumb-sized hit area

On now-playing-view -- the screen that exists so a phone has somewhere
to seek from -- the slider measured 261x6 on the reference device. Six
pixels is the whole of the drag target on the app's primary seeking
affordance, against the 44px floor the app set for itself in #56 and
holds to in the queue panel.

**The phone rule had never applied**, which is why the issue read as
"the thickening stops short" rather than "there is no thickening".
seek-bar's stylesheet asked for a 12px track below 599px and then set
6px in a plain `wa-slider` rule *written after it*. A media query adds
no specificity, so the plain rule won at every width: the source said
12 and the device said 6. That is index.css's documented rule -- "the
phone section is last on purpose" -- met inside a component's own
stylesheet, where nothing in any tier renders differently to say so.
The block is last now, and the 12px track it always asked for is real.

**And 12px is still under the floor**, so the target is built around
the painted track rather than by thickening it. The two are allowed to
differ and a slider is the clearest case where they should: a 44px
progress bar would be wrong-looking and would cost the album art the
vertical space #51 spent an issue recovering.

Two things about how it is built, both settled by measurement on the
device rather than by choosing a number.

**The padding goes on ::part(slider), not on the host.** That is the
issue's untested claim, and the answer is the pessimistic one: the
inner div is what carries the gesture -- it holds the listener and the
touch-action: none -- and it is exactly the host's size, so padding the
host would grow a box that does not take the press.

**The padding is asymmetric and the margins cancel it**, so the row does
not grow by the difference. The seek row is 19px -- its clocks, not the
track, decide that -- and the play button's top edge is 8px below it,
while `.art` above is a non-interactive div. A symmetric 44px target
reaches into the play button, and growing the row instead cost the art
25px of 143 when it was tried. So the target takes the space above.

Verified on the device at 424x439: hit area 261x44 where it was 261x6,
painted track 12px, seek row still 19px, album art still 143px, 7px of
clearance left under the play button, a press 26px above the track
seeks, and a hit test on the play button's top edge still reaches the
play button.

The desktop bottom bar is untouched: the rule is inside the phone query
and that instance is display:none below 600px anyway.

The test asserts the parsed stylesheet, on hover-affordance.test.ts's
precedent and with the same limitation stated -- no tier here lays out a
real wa-slider at a phone width, and a number measured on a phone is
not a number CI can assert. What it holds is the shape: that the phone
block is last, that padding plus track clears 44, that the margins
cancel the padding, and that the growth is upward. All four are
invisible on a desktop, and the first is exactly what a tidy-up undoes.

Closes #187

frontend/src/components/audio-player/seekbar/seek-bar.ts

@@ -45,18 +45,6 @@ export class SeekBar extends LitElement {
   private showRemaining: boolean = true;
 
   static override styles = [designTokens, waSliderLabel, css`
-    /* 12px below the phone breakpoint. The bottom bar's seek bar is
-       display:none there (016 B2 phase 1), so the only instance a
-       viewport media query can reach at that width is the full-screen
-       now-playing view's -- which is exactly the one a thumb uses.
-       The track size lives on wa-slider inside this shadow root, so a
-       custom property set by the host would not reach it. */
-    @media (max-width: 599px) {
-      wa-slider {
-        --track-size: 12px;
-      }
-    }
-
     wa-slider {
       --track-size: 6px;
       flex: 1;
@@ -80,6 +68,57 @@ export class SeekBar extends LitElement {
       background: var(--yj-bg-base, black);
     }
 
+    /* The phone's seek bar, and this block is last on purpose.
+
+       A media query adds no specificity, so this lived above the plain
+       "wa-slider" rule and lost to it at every width: the 12px track it
+       asks for had never once applied, and the bar measured 261x6 on
+       the device while the source said 12. That is index.css's rule
+       ("the phone section is last on purpose") met inside a component's
+       own stylesheet, and nothing renders differently in any tier here
+       to say so.
+
+       The bottom bar's seek bar is display:none below this width (016
+       B2 phase 1), so the only instance a viewport media query can
+       reach is the full-screen now-playing view's -- which is exactly
+       the one a thumb uses. The desktop bar keeps its 6px, where a
+       mouse is precise and the thickness is right.
+
+       The painted track and the thing you can hit are allowed to
+       differ, and a slider is the clearest case where they should: 12px
+       is a progress bar you can see, and 44px is the app's touch floor
+       (#56). A 44px-*thick* bar would be wrong-looking and would cost
+       the album art the vertical space #51 spent an issue recovering.
+
+       Two things about how the target is built.
+
+       The padding goes on ::part(slider) rather than on the host,
+       because that inner div is what carries the gesture -- it has the
+       listener and the touch-action: none, and it is exactly the host's
+       size, so padding the host would grow a box that does not take the
+       press.
+
+       The padding is asymmetric and the margins cancel it, so the row
+       does not grow by the difference. Both halves are measured: the
+       seek row is 19px (its clocks, not the track, decide that) and the
+       play button's top edge is 8px below it, so the target takes the
+       space *above*, where .art is a non-interactive div. Growing the
+       row instead cost the art 25px of 143. Verified on the device at
+       424x439: hit area 44px, painted track 12px, row still 19px, art
+       still 143px, 8px of clearance left under the play button, a press
+       26px above the track seeks, and a hit test on the play button's
+       top edge still reaches the play button. */
+    @media (max-width: 599px) {
+      wa-slider {
+        --track-size: 12px;
+      }
+
+      wa-slider::part(slider) {
+        padding-block: 28px 4px;
+        margin-block: -28px -4px;
+      }
+    }
+
     #seek-bar-container {
       display: flex;
       justify-content: space-between;