fix: silent pipeline log and server indexing progress

Three issues caused the Logs view to appear blank during a real pipeline run:

1. `from run import step_fetch, step_link` was outside the worker's
   try/except/finally. An import failure silently killed the thread,
   leaving _pipeline_done uncalled and the Run button stuck disabled
   forever. Now wrapped in its own try/except that posts the error to
   the log and resets the UI.

2. `build_server_index` makes N sequential HTTP requests (one per mod
   folder's meta.cpp) with no output during the scan. Added an optional
   `progress_fn(current, total, name)` callback; step_fetch wires it to
   print progress every 25 folders so the log never goes silent.

3. No immediate feedback after clicking Start — the log was blank until
   the worker thread started printing. Now posts a "Pipeline started"
   banner from the main thread before the worker launches.

arma_modlist_tools/fetcher.py

@@ -81,7 +81,11 @@ def make_session(auth: tuple[str, str]) -> requests.Session:
     return s
 
 
-def build_server_index(base_url: str, auth: tuple[str, str]) -> dict:
+def build_server_index(
+    base_url: str,
+    auth: tuple[str, str],
+    progress_fn: "Callable[[int, int, str], None] | None" = None,
+) -> dict:
     """
     Scan the root of the file server and build mod lookup maps.
 
@@ -90,6 +94,9 @@ def build_server_index(base_url: str, auth: tuple[str, str]) -> dict:
 
     :param base_url: Root URL of the Caddy file server (trailing slash optional).
     :param auth: ``(username, password)`` tuple for HTTP Basic Auth.
+    :param progress_fn: Optional callback called as ``progress_fn(current, total, name)``
+                        after each folder is processed.  Use it to report progress without
+                        coupling the library to ``print`` or any specific I/O sink.
     :returns: Dict with keys:
 
               - ``by_steam_id`` — ``{steam_id: folder_url}``
@@ -100,11 +107,12 @@ def build_server_index(base_url: str, auth: tuple[str, str]) -> dict:
     root = base_url.rstrip("/") + "/"
     items = _list_dir(root, session)
     folders = [it for it in items if it.get("is_dir")]
+    total = len(folders)
 
     by_steam_id: dict[str, str] = {}
     by_name: dict[str, str] = {}
 
-    for folder in folders:
+    for i, folder in enumerate(folders, 1):
         name = folder["name"].strip("/")
         url = _folder_url(root, name)
         by_name[_normalize_name(name)] = url
@@ -118,6 +126,9 @@ def build_server_index(base_url: str, auth: tuple[str, str]) -> dict:
         except requests.RequestException:
             pass  # meta.cpp missing or unreachable — name-based fallback still works
 
+        if progress_fn is not None:
+            progress_fn(i, total, name)
+
     return {"by_steam_id": by_steam_id, "by_name": by_name, "folders": folders}