feat: theme UX improvements + 16 warm-toned presets

[broodcicada-glitch]

What

Adds 16 curated warm-toned dark theme presets plus theme UX improvements.

Themes

Velvet Crimson, Plum Bruise, Pearl White, Warm Amber, Rose Quartz, Deep Crimson, Burnt Amber, Dusk Magenta, Clay Earth, Twilight Violet, Blush Petal, Midnight Plum, Storm Indigo, Coral Blush, Dawn Flush, Spring Peach.

All color-scheme: dark, derived from VSCode community themes.

UX Improvements

• Mutual exclusion: dropdown preset selected = 3 system theme buttons deselect
• Delete confirmation: prompt before removing theme

Files: 17 changed (1 settings + 16 CSS)

[github-actions]

PR quality triage

Changed areas: area:desktop

CLI core policy: No CLI-core policy block detected.

Missing-test policy: Blocked by policy until a maintainer applies allow-missing-tests or matching tests are added.

Coverage baseline policy: No coverage-baseline policy block detected.

CLI core files:

• none

Coverage policy files:

• none

Required check plan:

• See PR Quality / scope-plan; it is the single source of truth for selected jobs.

Test coverage signals:

• BLOCKING unless allow-missing-tests is applied: Desktop product files changed without a desktop test file in the PR.

Risk notes:

• No special risk notes from changed paths.

Hard merge gates come from the deterministic GitHub Actions contract lanes above.