feat(accessibility): finish HC chrome rollout — home + settings panel borders

Continues the rollout from `c9af1ea` (modal scaffold) and
`d87761d` (tooltip + 3 panels). Tags the remaining 7 static-
border surfaces in the chrome so the HC chrome thread is
effectively complete:

- **`home_plugin.rs` × 3**: the home-screen Level/XP/Score
  summary row (line 842), the home-screen mode-selector
  buttons (line 945), the home-screen mode-hotkey chips
  (line 1158).
- **`settings_plugin.rs` × 4**: the card-back picker swatches
  (line 1952), the theme picker swatches (line 2093), the
  Sync Now button (line 2214), and the swatch glyph buttons
  (line 2274).

Pre-tagging audit: confirmed none of these sites have a
dynamic-paint system that would race the
`update_high_contrast_borders` system. `paint_action_buttons`
in `hud_plugin.rs` only paints entities tagged with the
`ActionButton` marker (HUD buttons only). The focus-overlay
system in `ui_focus.rs` spawns *separate* overlay entities for
focus indication, never mutating the original `BorderColor`.
Settings panel buttons / swatches use their own
`SettingsButton` enum for click routing; their `BorderColor`
is set at spawn time and not touched again.

After this commit, every `BorderColor::all(BORDER_SUBTLE)` site
in the chrome (excluding the dynamic-paint sites that are
intentionally skipped — HUD action buttons, modal buttons,
radial menu rim) carries a `HighContrastBorder` marker. The
HC thread for chrome borders is closed; the dynamic-paint
sites remain open for a future iteration that needs a
different shape (folding HC into the dynamic-paint logic, or
having HC consult hover/focus state).

1194 passing / 0 failing across the workspace (unchanged — no
new tests; the system-level lifecycle of `HighContrastBorder`
was already covered by the modal-scaffold scaffolding in
`c9af1ea`). Workspace clippy clean.

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

solitaire_engine/src/settings_plugin.rs

@@ -1950,6 +1950,7 @@ fn picker_row(
                     },
                     BackgroundColor(bg),
                     BorderColor::all(BORDER_SUBTLE),
+                    HighContrastBorder::with_default(BORDER_SUBTLE),
                 ))
                 .with_children(|b| {
                     let text_color = if is_selected { BG_BASE } else { TEXT_PRIMARY };
@@ -2091,6 +2092,7 @@ fn theme_picker_row(
                     },
                     BackgroundColor(bg),
                     BorderColor::all(BORDER_SUBTLE),
+                    HighContrastBorder::with_default(BORDER_SUBTLE),
                 ))
                 .with_children(|b| {
                     spawn_thumbnail_pair(b, entry.thumbnails.as_ref());
@@ -2212,6 +2214,7 @@ fn sync_row(parent: &mut ChildSpawnerCommands, status_text: &str, font_res: Opti
                 },
                 BackgroundColor(BG_ELEVATED_HI),
                 BorderColor::all(BORDER_SUBTLE),
+                HighContrastBorder::with_default(BORDER_SUBTLE),
             ))
             .with_children(|b| {
                 b.spawn((
@@ -2272,6 +2275,7 @@ fn icon_button(
             },
             BackgroundColor(BG_ELEVATED_HI),
             BorderColor::all(BORDER_SUBTLE),
+            HighContrastBorder::with_default(BORDER_SUBTLE),
         ))
         .with_children(|b| {
             b.spawn((Text::new(label.to_string()), glyph_font, TextColor(TEXT_PRIMARY)));