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

.planning/NOTES.md

@@ -3961,3 +3961,71 @@ The general rule for this repo's fixture library: it is deliberately
 full of edge cases (untagged, unicode, duplicates, extremes), so a spec
 that wants an *ordinary* track has to **say so** — filter on the
 property it depends on rather than slicing.
+## A nested rule starting with an element name is dropped on the phone (2026-08-20)
+
+`CLAUDE.md` records that the device renders in **Chrome 113**, which
+does not have relaxed CSS nesting (Chrome 120). The consequence is
+sharper than "some syntax is unavailable": a nested rule whose selector
+begins with a bare identifier is not a parse error you would notice, it
+is **silently dropped**.
+
+Three such rules were live in `frontend/index.css`, all inside
+`.bottom-bar`, and all therefore dead on the phone and only on the
+phone:
+
+```css
+.bottom-bar {
+    #track-info { p { … } }   /* the metadata's ellipsis */
+    now-playing { overflow: hidden; }
+    audio-player { margin: 0.5em 1em; }
+}
+```
+
+The first is the interesting one: it is the *ellipsis* on the bottom
+bar's track title and artist, so on the device that text has never
+truncated — the same class of fault as `now-playing`'s marquee, whose
+`text-overflow` sat on the wrong box and had never produced an ellipsis
+in any mode. Both are invisible to every assertion and visible in a
+screenshot.
+
+`& p`, `& now-playing`, `& audio-player` are valid in both, so the fix
+is one character per rule. What is worth keeping is the rule of thumb:
+**inside a nested block, always write `&`** — and note that a rule
+inside `@media` is *not* nested, so `@media … { bottom-nav { … } }`
+elsewhere in that file is fine and needs nothing.
+
+`make css-check` does not catch this (it looks for backticks that end a
+tagged template early). Filed as an issue: the check is the natural
+place for it, being the same shape of trap — a silent, phone-only,
+screenshot-only failure.
+
+## Centring a bar costs the control in the middle of it (measured 2026-08-20)
+
+#23 asks for the transport centred in the bottom bar. The obvious
+implementation — make the outer two grid tracks the same width, so the
+middle is centred by construction — is right, and the first cut of it
+was a regression, because "the same width" was taken to mean *the
+metadata's* width on both sides.
+
+Measured at 800px, with the seek bar's own track:
+
+| layout | seek track | transport column |
+|---|---|---|
+| `320px 1fr auto` (before) | 257 | 407 |
+| both sides `--now-playing-width` | **61** | 179 |
+| both sides `min(--now-playing-width, 25%)` | 246 | 364 |
+
+At 200% text the middle row is worse still: 130 before, **0** with the
+uncapped sides. Centring is free at 1440 and expensive at 800, so a
+change checked only at a comfortable width looks perfect.
+
+The general form: **a symmetric layout reserves space on the side that
+does not need it.** The right-hand group here is ~141px (volume plus
+the queue button) and was being given 320 to keep the arithmetic
+symmetric. Cap the side tracks against the *bar*, not against their
+content, and the middle gets the difference.
+
+The spec that pins this is two assertions, not one, and that split is
+deliberate: an uncapped build is *perfectly centred* and fails only the
+seek-bar width, so a spec asserting centring alone would have passed
+the regression.