feat(player): centre the transport and show the volume inline

Two issues over one bar, because they are one relayout. #42's own
findings say so: giving wa-slider a label grows it 6px to 14px and
moves the transport, which is #23's subject, so doing them in sequence
means measuring the bar twice and throwing the first set away.

The bar was `320px 1fr auto`, so the transport sat in the middle of
what the metadata and the queue button did not use — its centre was
~140px right of the window's at every width. The outer two tracks are
the same expression now, so the middle is centred by construction.

The side width is the metadata's, capped at a quarter of the bar, and
the cap was measured as a regression before it was a decision:
reserving the full `--now-playing-width` on both sides is perfectly
centred and takes the seek bar's track from 257px to 61px at 800px, and
to 0 at 200% text. The control you drag was paying for the symmetry.
With the cap it is 246, which is parity. It is a `min()` rather than a
breakpoint because that variable is user state — the metadata has a
drag handle — and tying both sides to it is also what keeps dragging
meaningful; a plain `1fr … 1fr` centres just as well and silently makes
the handle a no-op.

The volume moved out of `audio-player` into the bar because the
transport column has to hold the transport and nothing else, and it
joins the queue button in one cell rather than a second column, since
the centring compares columns.

It is a slider by default and a popup by setting. The stored flag names
the *popup*, which is this config's polarity rule — the zero value has
to be the intended answer, so an existing config.toml gets the new
default with no migration. Inline, the icon is the mute toggle and is
named after that action rather than the state, because with the slider
beside it there is nothing to disclose; the component tier now covers
both presentations rather than whichever is default.

Three nested rules in this block began with a bare element selector,
which Chrome 120 relaxed and the phone's Chrome 113 **silently drops** —
including the ellipsis on the bar's own title and artist, which has
therefore never truncated on the device. They are `&`-prefixed now.
Filed as #154 for the class and for a check.

`bottom-bar.spec.ts` pins both halves separately on purpose: an
uncapped build is perfectly centred and fails only the seek-bar width,
so a spec asserting centring alone would have passed the regression
above. Both were verified by mutation.

Closes #23
Closes #42

backend/config/config.go

@@ -666,6 +666,49 @@ func (c *Config) SetAllowMeteredCatalogDownload(allow bool) error {
 	return nil
 }
 
+// GetPopupVolume reports whether the bottom bar's volume control is a
+// click-to-open popup rather than an inline slider (#42).
+func (c *Config) GetPopupVolume() bool {
+	if c.General == nil {
+		return false
+	}
+
+	return c.General.PopupVolume
+}
+
+// SetPopupVolume saves the volume control's presentation.
+//
+// Nothing to validate: both values are legal at every width, and the
+// frontend additionally stands the inline slider down below the phone
+// breakpoint whatever this says, because that is about room rather than
+// about preference.
+func (c *Config) SetPopupVolume(popup bool) error {
+	if c.General == nil {
+		c.General = &GeneralConfig{}
+		c.General.ApplyDefaults()
+	}
+
+	c.General.PopupVolume = popup
+
+	if err := c.Save(); err != nil {
+		return fmt.Errorf(
+			"could not save config: %w", err,
+		)
+	}
+
+	events.Emit(
+		c.ctx,
+		events.GeneralConfigChanged,
+		map[string]any{
+			"PopupVolume": popup,
+		},
+	)
+
+	c.logger.Info("volume control presentation updated", "popup", popup)
+
+	return nil
+}
+
 // GetViewVisibility reports which primary views the sidebar should
 // show, answered for every known view rather than only the ones the
 // config mentions -- so the frontend filters on a value and never has