fix(client): improve sandbox cold-start reliability and project sync for v0.0.2

[rcyw]

Summary

This PR improves Deck desktop startup reliability and first-run UX for local sandbox cold starts, and aligns landing/demo assets + release docs for v0.0.2.

What changed

1. Client startup and project detection

• Wait for OpenCode health before reporting local sandbox start success.
• Move local project detection ownership to Rust (/path-based) with lifecycle events and retry command.
• Add configurable startup timeouts:
  • DECK_SANDBOX_OPENCODE_HEALTH_TIMEOUT_MS (default 60000, clamped 5000-300000)
  • DECK_SANDBOX_PROJECT_PATH_TIMEOUT_MS (default 120000, clamped 5000-300000)
• Prevent remote profiles from being gated by local detecting-project logic.
• Fix transient status flicker (running briefly reverting to checking).

2. Startup performance and stability

• Avoid redundant health/path sync behavior during startup.
• Defer non-critical startup queries and reduce first-load request pressure.
• Keep project sync deduped/scoped and reduce repeated retry log noise.

3. UI / logging polish

• Add clearer cold-start phase hints and elapsed-time messages.
• Fix nested button markup warning in log viewer row.

4. Landing/demo/docs

• Standardize demo media pipeline around apps/landing/public/demo.mp4 + ffmpeg-generated assets.
• Add apps/landing/scripts/generate-demo-assets.sh and assets:demo script.
• Update root README media references to landing assets and remove old duplicate media path.
• Move changelog entries from Unreleased to 0.0.2.

Breaking changes

• None.

Validation

• cargo check --manifest-path apps/client/src-tauri/Cargo.toml
• pnpm --filter @cofy-x/deck-app run typecheck
• Targeted eslint checks for touched client files
• Landing asset generation/build verified

Release impact

• Recommended release version: v0.0.2 (patch).