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)

src/config_manager.py

@@ -190,8 +190,11 @@ class Config:
     def _relative_path(self, path: str) -> str:
         """Convert an absolute path to a relative path from the config file.
 
+        Uses :func:`os.path.relpath` which works across filesystem
+        boundaries (unlike :meth:`Path.relative_to`).
+
         Args:
-            path: Absolute path string.
+            path: Absolute or relative path string.
 
         Returns:
             Relative path string, or the original if conversion fails.
@@ -201,7 +204,8 @@ class Config:
         try:
             target = Path(path)
             if target.is_absolute():
-                return str(target.relative_to(self._config_path.parent))
+                base = str(self._config_path.parent)
+                return os.path.relpath(str(target), base)
         except (ValueError, OSError):
             pass
         return path
@@ -219,6 +223,34 @@ class Config:
             return Path()
         return (self._config_path.parent / relative_path).resolve()
 
+    def validate_paths(self) -> list[str]:
+        """Validate file paths in config and clear those that don't exist.
+
+        Checks each file path field, resolves it against the config
+        directory, and clears it if the target file does not exist
+        on the current filesystem.
+
+        Returns:
+            List of field names that were cleared.
+        """
+        cleared: list[str] = []
+        path_fields = [
+            "bootloader_bit_path",
+            "bootloader_elf_path",
+            "bootloader_bin_path",
+            "fsbl_elf_path",
+            "firmware_bin_path",
+        ]
+        for attr in path_fields:
+            val = getattr(self, attr, "")
+            if not val:
+                continue
+            resolved = self.resolve_path(val)
+            if not resolved.is_file():
+                setattr(self, attr, "")
+                cleared.append(attr)
+        return cleared
+
     @property
     def config_path(self) -> Path | None:
         """Return the path to the config file on disk."""