fix(passage): let the stimulus tabs follow the theme PIE-818

[chillenious]

The paired-passage tabs stayed white with black ink on every color scheme while the
passage body beneath them went dark, because the tab fill, the tab ink and the selection
indicator came from MUI's palette (theme.palette.common.white / .black), which does not
follow --pie-*. They now read color.background() and color.text() from
@pie-lib/render-ui, which resolve to var(--pie-background, ...) and
var(--pie-text, ...) -- the same tokens the passage body already uses, so the tab now
carries the colour of the panel it opens.

The two #D9DADA borders -- the tab outline and the strip's bottom rule -- become
color.borderGray(). The literal measured roughly 1.2:1 against the dark schemes'
surfaces, and the tab outline is the only thing separating an unselected tab from the strip,
so it was effectively absent exactly where it did the most work. --pie-border-gray is
stepped to the 3:1 non-text minimum in every built-in scheme.

The strip behind the tabs keeps a host-supplied --pie-passage-header-background
unchanged on every scheme, which is deliberate: Knowledge Checks' pale green-blue is a
client decision and not a scheme's business to override. What changes is the no-host
fallback, from a white literal to color.backgroundDark(), so the strip follows the theme
instead of staying white over a dark body. That covers hosts which never opt in, including
the section player when its own card-header token is unset.

Both theme.palette.common.* lines carried "replace with color.background() once PD-2801
is DONE". PD-2801 is about adding background styling to OT's color-contrast class -- the
mechanism pie-theme superseded -- and has been Blocked since 2023, so it no longer gates
this.

Not changed: the selected indicator stays color.tertiary() and the MUI indicator bar
stays color.white(). Both already resolve through tokens every scheme sets, --pie-white
being remapped to the scheme's own background rather than to white.

Verification

Builds and lints clean. Not verified in a rendered dark scheme: reproducing the tabs needs a
passage-pair item from the datastore (E266912 in the ticket), which needs a session, and this
package has no standalone demo page with a pair. Someone with a dev login should walk the
repro in the ticket before this is called done.

Not in scope

Heather's Figma also gives the unselected tab a distinct darker fill, where the component
still distinguishes selection by .passage-label opacity plus the underline. That is a design
refinement rather than a theming defect, so it is left alone here.