feat: add system theme option with an accessible theme menu

[justraman]

Closes #112

After first use the theme toggle became a binary light/dark switch and
stopped following OS theme changes. This replaces the cycling button
with a small popover menu offering Light / Dark / System, where System
tracks prefers-color-scheme live.

[github-actions]

⚡ Performance Report

⚠️ No baseline found on main. This PR's results are recorded but cannot be compared.
Merge to main to establish a baseline.

[github-actions]

Bundle Size Report

Chunks over 500 KB:

File	Raw	Brotli	Gzip
host/assets/paseo.smol-DboPaEh1.json	1.84 MB	941.7 KB	1019.4 KB
host/assets/paseo-people-next.smol.json	3.36 MB	1.68 MB	1.82 MB
host/assets/previewnet.smol.json	1.88 MB	181.4 KB	353.0 KB
host/assets/smoldot.js	2.98 MB	2.22 MB (+52 B)	2.23 MB
host/assets/smoldot_worker.js	2.95 MB	2.21 MB	2.22 MB
host/assets/wasm/web/truapi_server_bg.wasm	1.97 MB	623.3 KB	818.7 KB
Total	15.97 MB (+13.3 KB)	8.13 MB (+2.3 KB) (-49%)	8.76 MB (+2.7 KB)

All files

File	Raw	Brotli	Gzip
host/.well-known/apple-app-site-association	738 B	738 B	738 B
host/.well-known/assetlinks.json	1.3 KB	317 B	391 B
host/assets/bridge.js	68.9 KB	18.8 KB (+36 B)	21.6 KB (+2 B)
host/assets/browser.js	22.9 KB	7.6 KB (-2 B)	8.6 KB (+2 B)
host/assets/client.js	100.1 KB	29.4 KB (+10 B)	32.4 KB (+4 B)
host/assets/dist.js	39.0 KB	12.8 KB (-41 B)	14.6 KB (+1 B)
host/assets/dotli-debug-bus.js	710 B	710 B	710 B
host/assets/get-sync-provider.js	2.8 KB	1.1 KB (+1 B)	1.2 KB (+2 B)
host/assets/hex.js	154 B	154 B	154 B
host/assets/index.js	186.5 KB (+1.6 KB)	46.9 KB (+362 B)	56.0 KB (+485 B)
host/assets/index.css	45.9 KB (+725 B)	7.2 KB (+102 B)	8.1 KB (+127 B)
host/assets/manifest.js	22.5 KB	7.2 KB (-4 B)	7.9 KB (+2 B)
host/assets/panel.js	72.9 KB	19.8 KB (+9 B)	22.4 KB (+1 B)
host/assets/paseo.smol-DboPaEh1.json	1.84 MB	941.7 KB	1019.4 KB
host/assets/paseo-people-next.smol.json	3.36 MB	1.68 MB	1.82 MB
host/assets/paseo.smol.json	25.4 KB	4.9 KB	5.6 KB
host/assets/previewnet.smol.json	1.88 MB	181.4 KB	353.0 KB
host/assets/resolve.js	128 B	128 B	128 B
host/assets/rpc-resolve.js	2.4 KB	1.0 KB (-1 B)	1.1 KB (+2 B)
host/assets/shared-mode.js	1.8 KB	744 B (+1 B)	845 B
host/assets/smoldot.js	2.98 MB	2.22 MB (+52 B)	2.23 MB
host/assets/smoldot_worker.js	2.95 MB	2.21 MB	2.22 MB
host/assets/src.js	1.8 KB	849 B (-6 B)	946 B (+2 B)
host/assets/styles.css	15.1 KB	3.2 KB	3.8 KB
host/assets/wasm/web/README.md	10.9 KB	10.9 KB	10.9 KB
host/assets/wasm/web/package.json	371 B	371 B	371 B
host/assets/wasm/web/truapi_server.d.ts	6.9 KB	6.9 KB	6.9 KB
host/assets/wasm/web/truapi_server.js	35.6 KB	6.3 KB	7.2 KB
host/assets/wasm/web/truapi_server_bg.wasm	1.97 MB	623.3 KB	818.7 KB
host/assets/wasm/web/truapi_server_bg.wasm.d.ts	2.5 KB	2.5 KB	2.5 KB
host/assets/web.js	13.2 KB	3.5 KB	4.0 KB
host/assets/worker-runtime.js	6.3 KB (+6.2 KB)	1.6 KB (+1.5 KB)	1.8 KB (+1.7 KB)
host/assets/worker-runtime.js	106 B	106 B	106 B
host/assets/ws.js	23.1 KB	7.5 KB (+12 B)	8.2 KB (+4 B)
host/dotli.png	11.5 KB	11.5 KB	11.5 KB
host/favicon.svg	1.8 KB	1.8 KB	1.8 KB
host/host-sw.js	2.7 KB	1.1 KB (+16 B)	1.2 KB
host/icon-192.png	12.5 KB	12.5 KB	12.5 KB
host/icon-512.png	42.8 KB	42.8 KB	42.8 KB
host/index.html	23.9 KB (+4.0 KB)	4.6 KB (+222 B)	5.7 KB (+273 B)
host/manifest.webmanifest	441 B	441 B	441 B
host/workbox.js	14.8 KB	4.6 KB	5.1 KB
sandbox/app-sw.js	9.6 KB	3.1 KB (-1 B)	3.5 KB
sandbox/assets/bitswap-bridge.js	840 B	840 B	840 B
sandbox/assets/fetch.js	3.4 KB	1.2 KB	1.4 KB (+1 B)
sandbox/assets/index.css	45.9 KB (+725 B)	7.2 KB (+102 B)	8.1 KB (+127 B)
sandbox/assets/index.js	118.0 KB	33.7 KB (-36 B)	39.6 KB (+3 B)
sandbox/favicon.svg	1.8 KB	1.8 KB	1.8 KB
sandbox/index.html	1.7 KB	580 B (-1 B)	789 B (+2 B)
Total	15.97 MB (+13.3 KB)	8.13 MB (+2.3 KB) (-49%)	8.76 MB (+2.7 KB)

_{Commit: b4dbce8}

[github-actions]

❌ E2E Product suite failed on 277696f68b868ebd193b576f944aa5e173147162 — 38 passed, 23 failed, 1 skipped.

Failed tests:

• Product is ready
• Get Product Account
• Product Signer
• Account Connection Status
• Product Account Alias
• Request Login
• Get User Identity
• Subscribe Theme
• Derive Entropy
• Well-Known Chains
• StatementStore Allowance
• Bulletin Allowance
• Smart-Contract Allowance
• All Allowances
• String Write & Read
• Bytes Write & Read
• JSON Write & Read
• Clear
• Factory
• Feature Check
• Remote: HTTP/WS
• Remote: WebRTC
• Remote: Chain Submit

Logs: https://github.com/paritytech/dotli-community/actions/runs/30492123089
Artifacts: e2e-product-results (uploaded above) — open the failed test's trace.zip with npx playwright show-trace.

[BigTava]

No inline commets in tests. Pure // Given // When // Then please, so it easier for humans to see