feat: replace dashboard with Solid SPA

[EterUltimate]

Summary

• replace the legacy monolithic dashboard HTML with a bundled Solid.js/Vite SPA
• serve the SPA from /api/ and /api/index, with a 503 auto-refresh build hint when artifacts are missing
• add non-blocking dashboard autobuild fallback and artifact integrity checks
• keep bundled dashboard assets committed for no-Node plugin installs

Review notes

• removed obsolete dashboard.html/index.html/macos.html static pages
• kept login, change_password, and graph_share templates covered by existing static asset tests
• fixed the new jargon page to use backend-supported keyword/confirmed/pending query params

Validation

• python -m pytest tests/unit/test_frontend_builder.py tests/integration/test_webui_static_assets.py tests/integration/test_auth_blueprint.py
• pnpm typecheck
• pnpm test
• pnpm build

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