Fix sensing-server UI path resolution

[yash27-lab]

The server defaulted to ../../ui, which only worked from a specific working directory. Resolve the UI directory from common launch locations and warn clearly when assets are missing.

Add focused tests covering current-directory and executable-directory fallback discovery so the browser UI keeps working when the binary is launched outside the repo root.

[ruvnet]

Code Review — PR #227

Recommendation: MERGE (with minor suggestion)

Summary

This PR from @yash27-lab fixes issue #188 (visualization page 404) by adding intelligent UI path resolution to the sensing server. The server previously hardcoded ../../ui which only worked when launched from the exact build directory.

Changes

1. resolve_ui_path() searches multiple candidate locations:
   • The configured path directly
   • Relative to current working directory
   • Ancestor directories of cwd (looking for ui/)
   • Relative to the executable location
   • Ancestor directories of executable
   • Relative to CARGO_MANIFEST_DIR
2. Validates each candidate by checking for ui/index.html
3. Canonicalizes the resolved path
4. Falls back gracefully with a warning if no valid UI dir is found
5. Includes 3 focused unit tests covering the major resolution strategies

Security Assessment

• Path traversal: NOT a concern. The resolution only walks up ancestor directories and joins with "ui" — it never takes user-supplied path components from HTTP requests. The --ui-path CLI flag is operator-controlled, not user-facing.
• std::fs::canonicalize is used on resolved paths, which resolves symlinks and normalizes the path.
• The env!("CARGO_MANIFEST_DIR") fallback is compile-time only — safe.
• No XSS, injection, or unsafe code.

Conflict Assessment (ADR-069 through ADR-078)

• Minor conflicts expected with sensing-server/src/main.rs on main, since our recent work added new CLI arguments and modified the startup sequence. However, the changes are localized to:
  • Import additions (line 20-21)
  • A new function block inserted after Args struct
  • Minor changes in main() for path resolution
• These should be straightforward to resolve during merge.

Code Quality

• Good: Clean separation with resolve_ui_path_with_context() for testability (dependency injection of cwd/exe path).
• Good: Test coverage for the three main resolution strategies.
• Good: Informative log messages when path is resolved or when assets are missing.
• Minor: push_ancestor_ui_candidates could potentially generate many candidates for deeply nested paths, but this is a startup-only operation so performance is not a concern.

Suggestion

Consider adding a --no-ui flag or RUVIEW_NO_UI=1 env var for headless deployments that don't need static file serving, to suppress the warning.

Verdict: Clean fix with good test coverage. Safe to merge after resolving minor conflicts with current main.