feat(accessibility): wire BORDER_SUBTLE_HC into the modal scaffold

Resume-prompt Option E, part 2 of 2 — HC chrome borders. Pairs
with the reduce-motion gating in `ed152e2`.

v0.21.1 introduced `BORDER_SUBTLE_HC` (#a0a0a0) but never wired
it: the constant existed, no consumer used it. Spec at
`design-system.md` §Accessibility (#2) mandates outline boost
from `#505050` (BORDER_STRONG) to `#a0a0a0` under high-contrast
mode so panels and popovers stay legible on low-quality
displays.

### Architecture

- New `HighContrastBorder` component in `ui_theme` carrying a
  `default_color: Color` field that records the off-state colour
  the entity was spawned with. Tag any UI node where border
  legibility is accessibility-critical.
- New `update_high_contrast_borders` system in `settings_plugin`
  walks all tagged entities each Update tick, sets `BorderColor`
  to `BORDER_SUBTLE_HC` when `Settings::high_contrast_mode` is
  on, otherwise to `marker.default_color`. Compares against
  current `BorderColor` and only mutates when different so
  Bevy's change-detection doesn't trigger repaints every frame.

### Tagged in this commit

- The modal scaffold's card border (`ui_modal::spawn_modal`).
  This is the primary accessibility target — modals demand
  attention and a low-vision player needs to perceive the panel
  boundary. Default colour: `BORDER_STRONG` (#505050); HC
  variant: `BORDER_SUBTLE_HC` (#a0a0a0).

### Future scope

Other `BORDER_SUBTLE` / `BORDER_STRONG` consumer sites (help
panel, stats panel, tooltip, action buttons, settings rows,
etc.) can be tagged in follow-ups by adding
`HighContrastBorder::with_default(...)` to their spawn tuple.
The system handles any entity carrying the marker — no further
changes needed once a site is tagged. Started small here to
keep the commit reviewable and prove the architecture before
rolling out broadly.

Workspace clippy + cargo test --workspace clean. 1193 passing
(unchanged from prior — no new tests added; the system is
small enough that the running-game verification is the meaningful
check).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

solitaire_engine/src/ui_modal.rs

@@ -58,8 +58,9 @@ use crate::settings_plugin::SettingsResource;
 use crate::ui_theme::{
     scaled_duration, ACCENT_PRIMARY, ACCENT_PRIMARY_HOVER, ACCENT_SECONDARY, BG_BASE, BG_ELEVATED,
     BG_ELEVATED_HI, BG_ELEVATED_PRESSED, BG_ELEVATED_TOP, BORDER_STRONG, BORDER_SUBTLE,
-    MOTION_MODAL_SECS, RADIUS_LG, RADIUS_MD, SCRIM, TEXT_PRIMARY, TEXT_SECONDARY, TYPE_BODY_LG,
-    TYPE_CAPTION, TYPE_HEADLINE, VAL_SPACE_2, VAL_SPACE_3, VAL_SPACE_4, VAL_SPACE_5,
+    HighContrastBorder, MOTION_MODAL_SECS, RADIUS_LG, RADIUS_MD, SCRIM, TEXT_PRIMARY,
+    TEXT_SECONDARY, TYPE_BODY_LG, TYPE_CAPTION, TYPE_HEADLINE, VAL_SPACE_2, VAL_SPACE_3,
+    VAL_SPACE_4, VAL_SPACE_5,
 };
 
 // ---------------------------------------------------------------------------
@@ -230,6 +231,13 @@ where
                 Transform::from_scale(Vec3::splat(MODAL_ENTER_START_SCALE)),
                 BackgroundColor(BG_ELEVATED),
                 BorderColor::all(BORDER_STRONG),
+                // Honour `Settings::high_contrast_mode`: under HC the
+                // border boosts from `BORDER_STRONG` (#505050) to
+                // `BORDER_SUBTLE_HC` (#a0a0a0) so the modal panel
+                // edge stays clearly visible against the scrim and
+                // surface beneath. `update_high_contrast_borders` in
+                // `settings_plugin` does the per-frame swap.
+                HighContrastBorder::with_default(BORDER_STRONG),
             ))
             .with_children(build_card);
         })