fix(qt): table headers line up, the mini rail is centred, tooltips have a background

Three of the owner's visual reports, one theme: geometry that was derived
in two places and drifted.

The track table laid its header out from the LABELLED columns only, so
the heart and the overflow menu took no space there and every label sat
right of the column it names. Header and rows now come from one shared
set of widths, including the responsive arms -- if the header hid columns
on a different condition than the row, the drift would come back only on
narrow windows.

The collapsed rail goes back to Slint's 64px. It had been 50 by owner
request, and the measurement showed why that was the actual defect: the
rail centred its row at 25 while the header's own sidebar toggle, sitting
directly above it, centres at 30. Symmetric padding inside a 50px rail
still reads as misaligned because the eye compares the glyphs to the
header control, not to the rail; chasing it from inside bought the
alignment at the price of a visibly asymmetric hover pill. At 64 both
references land 2px apart and the padding is symmetric again. The row
stays a literal 34 rather than being derived from the width -- deriving
it is what coupled the two tokens and would have silently grown every row
to 48.

Tooltips: the port had none, while Slint has a shell-level overlay. This
adds the overlay rather than a Popup per call site.

cc

crates/qbz-qt/qml/theme/QbzTheme.qml

@@ -348,19 +348,29 @@ QtObject {
     readonly property int headerHeight: 42
     // Three-state sidebar rendered widths (ShellState.sidebar-rendered-width).
     readonly property int sidebarOpenWidth: 240
-    // Mini rail — DELIBERATE deviation from state.slint:4075 (64px), owner
-    // request: the 64px rail read as a wide empty gutter around a 32px row.
-    // rail = spacingSm + row + spacingSm, so the square mini row exactly
-    // fills the 8px-padded track (Sidebar.qml root padding = Sidebar.slint:
-    // 716-718). REVERT is TWO tokens, not one: putting 64 back here also
-    // makes sidebarMiniRow 48, so the rows would grow from 34 to 48 and stop
-    // matching Slint — set sidebarMiniRow to a literal 34 at the same time.
-    // (Adversarial review caught this comment claiming a one-token revert.)
-    readonly property int sidebarMiniWidth: 50
-    // The square row inside that rail (34px). Rows are `width: parent.width`,
-    // so this is only their HEIGHT; it exists as a token so the rail and the
-    // row can never drift apart.
-    readonly property int sidebarMiniRow: sidebarMiniWidth - 2 * spacingSm
+    // Mini rail — back to Slint's 64px (state.slint:4075) by owner decision,
+    // 2026-07-29, after the 50px deviation turned out to have a cost nobody
+    // had priced.
+    //
+    // The 50px came from an earlier owner request: the 64px rail read as a
+    // wide empty gutter. But 50 made two references irreconcilable — the rail
+    // centres its 34px row at 25, while the header's own sidebar toggle sits
+    // at centre 30 (spacingMd + half of 28). Five pixels apart, one directly
+    // above the other, which is what the owner saw and reported as "not
+    // centred": symmetric padding inside the rail still looked wrong because
+    // the eye lines the glyphs up against the header control, not against the
+    // rail. Slint never had the problem — at 64 the row centres at 32, two
+    // pixels off the header button and imperceptible. Chasing it from inside
+    // the rail (12/4 padding) bought the header alignment at the price of a
+    // visibly asymmetric hover pill; the root fix is the width.
+    readonly property int sidebarMiniWidth: 64
+    // The square row inside that rail. A LITERAL 34, not derived: at 50 the
+    // `width - 2 * spacingSm` formula happened to yield 34, which is why the
+    // two tokens were coupled — but Slint pins BOTH independently (rail 64px,
+    // row 34px at Sidebar.slint:575/652), so the same formula at 64 would give
+    // 48 and silently grow every row. Deriving it was the trap the previous
+    // comment warned about; pinning it is the fix.
+    readonly property int sidebarMiniRow: 34
     // Right queue/lyrics column width (AppShell).
     readonly property int queuePanelWidth: 300
     // Small now-playing bar total height (ShellState.npb-small-height,