feat(app): theme pack, theme picker, and dev library preview mode

[aaf2tbz]

Summary

Adds four new UI themes (Skeleton, Forest, Orange Peel, Dragonfruit), a theme-picker popover replacing the cycle button, per-theme nav icon sets, light/developer theme polish, and a METALSHARP_DEV_LIBRARY=1 developer flow (npm run preview:dev) that skips first-launch/migration gates straight to the library view. Also widens the dev-mode backend /status timeout so a first-run bottle scan no longer triggers a backend kill/restart loop that left the library empty.

Changes

• New themes in useTheme.ts + variables.css (full variable sets, same structure as existing themes); per-theme glass-header tints in base.css and sidebar backgrounds in Sidebar.vue. Shared glass blur values, sidebar widths, header sizing, and card hover scaling are unchanged — new themes supply tints only.
• Light theme polish: warmer surfaces, deeper accent contrast, frosted header tint, stronger card borders. Developer theme polish: reduced non-accent saturation, brighter dim text, buttons use the standard border-radius instead of pill shape.
• Theme picker popover on the sidebar-bottom button listing all 7 themes with lucide icons; button position/size unchanged.
• Per-theme nav icon maps (library / sharp / logs / refresh / start-wine-steam) for custom themes; dark/light keep the original icons.
• app/src/main/index.ts: METALSHARP_DEV_LIBRARY=1 appends skip-to=library to the loadFile query; App.vue consumes it, waits for backend readiness, then calls initApp() directly. New preview:dev npm script.
• rust-bridge.ts: dev-mode /status timeout raised 1500ms → 10000ms so ensureRunning does not restart a busy dev backend mid-scan.
• shims-vue.d.ts: ~icons/* module declaration for tsc.

PR Readiness (MANDATORY)

• Compatibility verified with at least one real game (game + launch method noted below)
• No hardcoded paths, secrets, or absolute /Users/... paths introduced
• Config/rules TOML validated if configs/mtsp-rules.toml or DLL maps changed
• Version triple (CMakeLists.txt, Cargo.toml, package.json, package-lock.json) in sync if version bumped
• Bottle/runtime migration and launch behavior preserved (rollback plan noted if changed)
• Docs / compatibility matrix updated for user-facing changes
• Regression test added for each bug fix

Bypassed intentionally via checklist-exception: UI/renderer-only change — no launch behavior, configs, or version bumps touched.

Local toolchain (run before push)

• TypeScript compiles if changed: cd app && npx tsc --noEmit
• Biome pass if TS/JS changed: cd app && npx @biomejs/biome ci src/ (4 pre-existing CSS specificity warnings, unchanged)
• Prettier check passes on changed files
• No hardcoded paths, secrets, or absolute /Users/... paths
• No new files added to the repo root

Test notes

Verified on macOS via npm run preview:dev (Electron, dev backend metalsharp-backend 0.56.8 on port 9276 against a real ~/.metalsharp library): app boots straight into the library view, real installed game cards load and render, all 7 themes apply correctly through the picker, sidebar/header glassmorphism and geometry intact in every theme. No games launched — UI-only change.

Risk

Low: renderer-only styling plus an opt-in dev env var. The one main-process behavior change (longer dev /status timeout) only affects unpackaged dev runs; production timeout unchanged. Rollback: revert this commit.