Improve learning layout, expand to 200 concepts, and add subscription previews - #7

Changes

The concept screen previously displayed automatic playback and manual controls together, while fixed GIF boxes could overflow. This change expands the main content width, separates example modes with vertical tabs, puts values above bracketed indices, fits GIF labels, and highlights current/changed state separately from visited state.

The catalog now contains 200 unique lessons: 47 foundational algorithms/data structures plus 153 focused concepts with authored principles and concrete worked cases. Focused lessons explicitly reuse a related foundational animation, prediction sequence, and coding exercise. The existing 47 complete coding problems remain unchanged. Complexity tables distinguish core operations and their assumptions from total exercise costs.

Authenticated /plans and /checkout pages preview the intended free/review subscription offering. Paid price and quota remain unspecified; no payment credentials are collected, no purchase request is made, and checkout is disabled.

Validation

• npm run check: typecheck/build passed; 61 unit/integration tests passed, 1 conditional PostgreSQL test skipped locally.
• npm run test:e2e: 22 browser tests passed, including all 200 concept paths, vertical tab keyboard interaction, code-route restoration, desktop/mobile overflow and payment non-mutation.
• Regenerated 188 animation assets (47 algorithms × two themes × GIF/PNG) and visually inspected representative array, FFT, trie and tree frames.

New screen views and example-mode switches use the existing validated GA4/Amplitude event pipeline. The final logging change also passed the 3 relevant analytics/preview browser tests.

Depends on PR #6; base is codex/learning-paths.

Deployment

• Cloud Run revision algostep-00015-5hs serves 100% of traffic.
• Final image: us-central1-docker.pkg.dev/algostep/algostep/api:9be5614f-43b7-4db3-84f6-6addbc6ede0d.
• Authenticated production UI verified the 200-concept catalog, exclusive example tabs, worked examples and complexity tables, /plans, and disabled /checkout.
• Final focused curriculum seeding uses two DB round trips instead of 306, preserves existing published rows, and passed six focused tests plus the final GitHub CI run.
• Production direct routes/assets/health passed; protected API routes still reject anonymous requests and missing API/assets remain 404.