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/settings_plugin.rs

@@ -34,9 +34,9 @@ use crate::ui_modal::{
 };
 use crate::ui_tooltip::Tooltip;
 use crate::ui_theme::{
-    BG_BASE, BG_ELEVATED, BG_ELEVATED_HI, BORDER_SUBTLE, RADIUS_SM, SPACE_2, STATE_SUCCESS,
-    TEXT_PRIMARY, TEXT_SECONDARY, TYPE_BODY, TYPE_BODY_LG, TYPE_CAPTION, VAL_SPACE_2, VAL_SPACE_3,
-    Z_MODAL_PANEL,
+    BG_BASE, BG_ELEVATED, BG_ELEVATED_HI, BORDER_SUBTLE, BORDER_SUBTLE_HC, HighContrastBorder,
+    RADIUS_SM, SPACE_2, STATE_SUCCESS, TEXT_PRIMARY, TEXT_SECONDARY, TYPE_BODY, TYPE_BODY_LG,
+    TYPE_CAPTION, VAL_SPACE_2, VAL_SPACE_3, Z_MODAL_PANEL,
 };
 
 /// Side length of a swatch button in the card-back / background pickers.
@@ -364,6 +364,7 @@ impl Plugin for SettingsPlugin {
                     update_anim_speed_text,
                     update_color_blind_text,
                     update_high_contrast_text,
+                    update_high_contrast_borders,
                     update_reduce_motion_text,
                     update_tooltip_delay_text,
                     update_time_bonus_multiplier_text,
@@ -637,6 +638,42 @@ fn update_high_contrast_text(
     }
 }
 
+/// Repaints `BorderColor` on every entity tagged with
+/// [`HighContrastBorder`] based on `Settings::high_contrast_mode`.
+/// Off → the marker's `default_color`; on → `BORDER_SUBTLE_HC`
+/// (`#a0a0a0`). Compares against the current border colour and
+/// only mutates when different so Bevy's change-detection
+/// doesn't trigger repaints every frame.
+///
+/// Spec at `design-system.md` §Accessibility (#2): under HC,
+/// outlines boost from `#505050` (BORDER_STRONG) to `#a0a0a0` so
+/// modal panels, popover edges, and focus-ring carriers stay
+/// legible on low-quality displays / for low-vision users.
+///
+/// Tagged sites in v0.21.x: the modal scaffold's card border
+/// (`ui_modal::spawn_modal`). More sites can be tagged in
+/// follow-ups by adding `HighContrastBorder::with_default(...)`
+/// to their spawn tuple.
+fn update_high_contrast_borders(
+    settings: Res<SettingsResource>,
+    mut borders: Query<(&HighContrastBorder, &mut BorderColor)>,
+) {
+    let high_contrast = settings.0.high_contrast_mode;
+    for (marker, mut border) in borders.iter_mut() {
+        let target = if high_contrast {
+            BORDER_SUBTLE_HC
+        } else {
+            marker.default_color
+        };
+        // Only mutate when actually different — avoids per-frame
+        // change-detection churn. `border.left` is representative
+        // because every tagged site uses `BorderColor::all(...)`.
+        if border.left != target {
+            *border = BorderColor::all(target);
+        }
+    }
+}
+
 fn update_reduce_motion_text(
     settings: Res<SettingsResource>,
     mut text_nodes: Query<&mut Text, With<ReduceMotionText>>,