fix(qt): cortinilla geometry - NPB-aware cap, scrim inset, row width, scroll base

Five geometry defects, all latent while the Qobuz-only panel held at most 14
rows and never scrolled, and all reachable the moment the local sections land.

  - The height cap hardcoded 42 for the now-playing bar in every mode. The
    bar is 42 only in NPB mode 2; everywhere else it is 112, so the panel
    could run up to 70px into it. The cap is now mode-aware, reading the same
    theme tokens AppShell uses. The header height literals in the scrims and
    the panel anchor became theme.headerHeight at the same time.

  - The click-outside scrims left a gap for the search field computed without
    the 60px the field gives up when the section nav sits in the header, so
    with nav-in-header there was a 30px dead strip on each side of the field
    that neither dismissed the panel nor reached the input. The scrim now
    tracks HeaderBar's own width rule exactly.

  - Rows were 12px wider than their block: a QML Column's padding does not
    size its children, and parent.width ignored it. clip hid the overflow,
    but the highlight ran flush to the right edge while staying inset 6px on
    the left.

  - flat_index_content_y started at 6.0, mirroring the panel's 6px top
    padding - which is a Flickable VIEWPORT margin, not content space. Every
    row was reported 6px low, so scroll-into-view over-scrolled going down
    and clipped the row's top going up. The immersive twin, written later
    against the same layout, already started at 0.0.

    Two unit tests now pin the arithmetic. There were none for the desktop
    arm, which is why the 6.0 survived; the immersive arm had its equivalent
    and was correct.

  - The scrollbar used the primitive's default 14px gutter where the
    reference overrides it to 10.

Plus the "Enter" affordance glyph (U+23CE + one space -> U+21B5 + two, as in
the reference) and the panel drop shadow, which reuses the immersive twin's
idiom - a mirror rectangle blurred by MultiEffect, skipped when there are no
shaders - rather than introducing a second shadow mechanism. NavFlyout still
has none and says so; that is its own parity pass.

AppShell's comment claiming the cortinilla is the "LAST child so it renders
above every surface" is corrected: four siblings are declared after it, and
that is fine.

Contract: qbz-nix-docs/qt-frontend/2026-08-03-cortinilla-parity, commit C4
(D-BUG-3/4/5/7, rows A5/A7/A10/A12/A13/A19/A28/A2, UT-1).

Gate: build clean, 258/258 tests (2 new), both QML audits OK, offscreen
smoke 0 complaints + home published.

cc

crates/qbz-qt/qml/shell/HeaderBar.qml

@@ -745,7 +745,7 @@ Rectangle {
             anchors.right: parent.right
             anchors.rightMargin: 10
             height: parent.height
-            text: "⏎ " + QbzSession.tr("Enter", QbzSession.trRev)
+            text: "↵  " + QbzSession.tr("Enter", QbzSession.trRev)
             color: theme.textMuted
             font.pixelSize: 12
             verticalAlignment: Text.AlignVCenter