feat(desktop): flag already-installed themes in the install pickers

[OutThisLife]

Summary

Going to Cmd-K → "Install theme…" listed VS Code Marketplace themes with no indication you already had them, and clicking one re-downloaded + re-installed a theme you already owned. The Appearance settings grid already detected installed themes — but by parsing theme descriptions inline on every render, plumbing that never reached the palette.

This lifts that detection into one reactive source and reuses it on both surfaces:

• $marketplaceInstalls (computed over $userThemes): extensionId → installed theme, derived once via marketplaceIdOf and memoized — instead of rebuilding a Set on every render.
• Both install surfaces now mark owned rows as installed (the ✓ "installed" state) and, on click, re-activate the installed theme instead of re-fetching it.
• Removes the duplicated description-parsing in settings and the per-session "installed here" state in both surfaces. The store is the source of truth now, so themes installed in a previous session show as installed too (the old session-flag missed those).

Why this shape

• DRY / "use the plumbing": one selector in themes/user-themes.ts (where installed themes already live) replaces two ad-hoc derivations.
• Performance: memoized computed recomputes only when $userThemes changes, vs. a fresh Set/parse per render; clicking an installed theme is now a store lookup + activate instead of a network download + unzip + convert.

Files

• themes/user-themes.ts — marketplaceIdOf() + $marketplaceInstalls selector.
• app/command-palette/marketplace-theme-page.tsx — mark installed; activate-on-click.
• app/settings/appearance-settings.tsx — consume the shared selector; drop inline parsing + session state.
• themes/user-themes.test.ts — coverage for the new selector.

Test plan

• vitest run --environment jsdom themes/user-themes.test.ts — 9 passed
• tsc -p . --noEmit — clean
• eslint on all touched files — clean
• Manual: install a theme from the Cmd-K palette → it shows ✓ installed; reopen the palette and the same theme reads installed; clicking it just re-activates (no network). Same row state mirrored in Appearance settings.