Apply a linked theme's palettes to scheme islands stamped inside a card

[burieberry]

A data-theme="dark" (or .dark) stamped inside a themed card's template only got the theme.css dark defaults: the style container query that applies a theme's darkModeVariables resolves against the scope element's ancestors, so it never saw an island below it. Themed instances rendered dark Boxel surfaces with the theme's light accents.

• themeScopedCss re-emits both palettes on scheme islands via an @scope block bounded at the next themed card, so a nested card's islands keep its own theme.
• theme.css repeats the chrome-knob boundary reset on islands, so buttons inside an island hover and border with the theme's tokens like the rest of the card, instead of snapping back to Boxel chrome colors.
• New acceptance suite covers islands, nested islands by attribute and by class, and a nested themed card's own island.

Fixes CS-12988.

🤖 Generated with Claude Code

[github-actions]

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Host Test Results

    1 files      1 suites   2h 8m 19s ⏱️
4 918 tests 4 904 ✅ 14 💤 0 ❌
4 933 runs  4 919 ✅ 14 💤 0 ❌

Results for commit 88fe61d.

Realm Server Test Results

    1 files    244 suites   1h 27m 7s ⏱️
3 618 tests 3 618 ✅ 0 💤 0 ❌
3 669 runs  3 669 ✅ 0 💤 0 ❌

Results for commit 88fe61d.