fix: config save broken (relative_to→relpath), startup validation, auto-create, untrack config.yaml

- _relative_path(): replace Path.relative_to() with os.path.relpath()
  (ValueError crossing filesystems silently returned absolute paths)
- _sync_config_from_ui(): always derive relative from get_path() absolute,
  never trust stale FileSelector._relative_path after Browse
- validate_paths(): check all file paths exist on startup, clear missing
- Clear UI selectors after validation to prevent re-save of stale paths
- _load_config(): auto-create blank config.yaml when missing on disk
- .gitignore: add config.yaml (user-specific, must not be tracked)

.gitignore

@@ -33,7 +33,8 @@ htmlcov/
 # ── Opencode (internal tooling) ──────────────────────────────
 .opencode/
 
-# ── Config overrides (user-specific) ─────────────────────────
+# ── Config (user-specific, auto-generated if missing) ─────────
+config.yaml
 config/zynq_flasher.yaml
 
 # ── Logs ─────────────────────────────────────────────────────