fix: improve build_frontend reliability and cross-platform compatibility

Addresses concerns from PR #76 code review:

- Add exception handling for stat() calls to prevent crashes from race
  conditions when files are deleted/modified during iteration
- Add 2-second timestamp tolerance for FAT32 filesystem compatibility
  (FAT32 has 2-second mtime precision on USB drives/SD cards)
- Add config file checks (package.json, vite.config.ts, tailwind.config.ts,
  tsconfig.json, etc.) that also require rebuilds when changed
- Add logging to show which file triggered the rebuild for debugging

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

start_ui.py

@@ -141,38 +141,90 @@ def install_npm_deps() -> bool:
 
 
 def build_frontend() -> bool:
-    """Build the React frontend if dist doesn't exist or is stale."""
+    """Build the React frontend if dist doesn't exist or is stale.
+
+    Staleness is determined by comparing modification times of:
+    - Source files in ui/src/
+    - Config files (package.json, vite.config.ts, etc.)
+    Against the newest file in ui/dist/
+
+    Includes a 2-second tolerance for FAT32 filesystem compatibility.
+    """
     dist_dir = UI_DIR / "dist"
     src_dir = UI_DIR / "src"
 
+    # FAT32 has 2-second timestamp precision, so we add tolerance to avoid
+    # false negatives when projects are on USB drives or SD cards
+    TIMESTAMP_TOLERANCE = 2
+
+    # Config files that should trigger a rebuild when changed
+    CONFIG_FILES = [
+        "package.json",
+        "package-lock.json",
+        "vite.config.ts",
+        "tailwind.config.ts",
+        "tsconfig.json",
+        "tsconfig.node.json",
+        "postcss.config.js",
+        "index.html",
+    ]
+
     # Check if build is needed
     needs_build = False
+    trigger_file = None
 
     if not dist_dir.exists():
         needs_build = True
+        trigger_file = "dist/ directory missing"
     elif src_dir.exists():
         # Find the newest file in dist/ directory
         newest_dist_mtime = 0
         for dist_file in dist_dir.rglob("*"):
-            if dist_file.is_file():
-                file_mtime = dist_file.stat().st_mtime
-                if file_mtime > newest_dist_mtime:
-                    newest_dist_mtime = file_mtime
+            try:
+                if dist_file.is_file():
+                    file_mtime = dist_file.stat().st_mtime
+                    if file_mtime > newest_dist_mtime:
+                        newest_dist_mtime = file_mtime
+            except (FileNotFoundError, PermissionError, OSError):
+                # File was deleted or became inaccessible during iteration
+                continue
 
-        # Check if any source file is newer than the newest dist file
         if newest_dist_mtime > 0:
-            for src_file in src_dir.rglob("*"):
-                if src_file.is_file() and src_file.stat().st_mtime > newest_dist_mtime:
-                    needs_build = True
-                    break
+            # Check config files first (these always require rebuild)
+            for config_name in CONFIG_FILES:
+                config_path = UI_DIR / config_name
+                try:
+                    if config_path.exists():
+                        if config_path.stat().st_mtime > newest_dist_mtime + TIMESTAMP_TOLERANCE:
+                            needs_build = True
+                            trigger_file = config_name
+                            break
+                except (FileNotFoundError, PermissionError, OSError):
+                    continue
+
+            # Check source files if no config triggered rebuild
+            if not needs_build:
+                for src_file in src_dir.rglob("*"):
+                    try:
+                        if src_file.is_file():
+                            if src_file.stat().st_mtime > newest_dist_mtime + TIMESTAMP_TOLERANCE:
+                                needs_build = True
+                                trigger_file = str(src_file.relative_to(UI_DIR))
+                                break
+                    except (FileNotFoundError, PermissionError, OSError):
+                        # File was deleted or became inaccessible during iteration
+                        continue
         else:
             # No files found in dist, need to rebuild
             needs_build = True
+            trigger_file = "dist/ directory is empty"
 
     if not needs_build:
         print("  Frontend already built (up to date)")
         return True
 
+    if trigger_file:
+        print(f"  Rebuild triggered by: {trigger_file}")
     print("  Building React frontend...")
     npm_cmd = "npm.cmd" if sys.platform == "win32" else "npm"
     return run_command([npm_cmd, "run", "build"], cwd=UI_DIR)