fix(web): restore Settings subtab-pill hover contrast in dark theme (#1815)

The .subtab-pill button hover rule hard-coded `rgba(255,255,255,0.6)` as the
hover background. In light theme this composites to a near-white wash that
sits cleanly under `--text`, but in dark theme it overlays a translucent
white on the dark panel and produces a bright, near-white surface beneath
the same light `--text`, dropping contrast to ~1.87 (well below WCAG AA 4.5).

The reporter (#1795, on 0.6.0) called this out on five Settings surfaces.
Four of them — BYOK / Appearance / Notifications / protocol-chip — were
incidentally fixed by the c5d77a03 dark-palette refresh because they use
the .seg-control / .seg-btn hover path, not the .subtab-pill path. The
Pet → pet source tabs (Built-in / Custom / Community) still reproduce on
0.7.0 because they use .subtab-pill, which never got a theme-aware hover.

Switch the hover background to `var(--bg-muted)`, an existing token that
mirrors `--bg-panel`'s active state shape (one step lighter than
`--bg-subtle`) and is defined per-theme. Composited contrast becomes
~9.4 in dark and ~14.3 in light against `--text`, comfortably above AA.

The same .subtab-pill rule is shared by RoutinesSection and DesignsTab, so
they pick up the fix as a side effect of fixing the source class.

Anchored by a Playwright regression test under e2e/ui/ that asserts WCAG
AA across both themes for the Pet source tabs and the seg-btn surfaces.
The test goes red on this commit's parent (Pets/dark, ratio 1.87) and
green here (all four assertions pass).

apps/web/src/index.css

@@ -8123,7 +8123,7 @@ button.connector-action.is-loading {
   font-weight: 500;
   white-space: nowrap;
 }
-.subtab-pill button:hover:not(.active) { background: rgba(255,255,255,0.6); border-color: transparent; color: var(--text); }
+.subtab-pill button:hover:not(.active) { background: var(--bg-muted); border-color: transparent; color: var(--text); }
 .subtab-pill button.active {
   background: var(--bg-panel);
   color: var(--text);