fix: preserve playback and refine subscriptions

[Priveetee]

Summary

• pass the remote YouTube login enablement flag through the server compose service
• align stack database environment defaults around POSTGRES_* while preserving existing server and downloader mappings
• split subscriptions into separate Videos and Channels views with lazy channel rendering and proxied channel avatars
• improve mobile history cards and move discovery toggles into Settings > Interface
• preserve video playback position and playing state when returning to a browser tab

Production behavior

• production compose can enable remote YouTube login through the server service environment, refs #125: YOUTUBE_REMOTE_LOGIN_ENABLED=true has no effect
• stack setup now writes POSTGRES_DB, POSTGRES_USER, and POSTGRES_PASSWORD as the canonical database variables
• existing DATABASE_* and downloader DB_URL overrides remain supported by compose defaults
• Subscriptions keeps videos on the main page and exposes followed channels at /subscriptions/channels
• channel avatars from YouTube image hosts are proxied and fall back cleanly when unavailable
• Settings labels Home as Interface while keeping the existing section key compatible
• returning to a playing watch tab no longer forces a fresh playback start from the beginning, refs #129: Playback stops and video returns to beginning when going back to a playing browser tab

Deployment requirements

• no new frontend dependencies are added
• production can keep existing DATABASE_* values, or move to POSTGRES_* values during normal environment maintenance
• remote YouTube login still requires the matching production environment values and backend support already deployed for the feature

Runtime verification

• compose configuration was validated for the production and dev stack files
• player focus and visibility handling was validated through TypeScript and build checks after the playback guard change
• subscriptions route generation was refreshed after adding /subscriptions/channels

Tests

• bun run --cwd apps/web tsc -b --force
• bun run check
• bun run knip
• bun run sherif
• bun run build
• git diff --check
• docker compose -f docker-compose.yml config --quiet
• docker compose -f docker-compose.dev.yml config --quiet

Rollback

• revert the playback return guard commit if watch playback focus handling regresses
• revert the subscriptions UI commits if subscriptions navigation or channel rendering regresses
• revert the stack environment commit if production compose environment resolution regresses