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.

gui/app.py

@@ -143,11 +143,22 @@ class ArmaModManagerApp(ctk.CTk):
         self._pipeline_running = True
         self._get_dashboard().set_pipeline_ui(running=True)
         self.navigate_to("Logs")
+        # Post an immediate banner so the log is never blank after clicking Start.
+        _sep = "=" * 50
+        self.post_log(f"\n{_sep}\n  {t('pipeline.starting')}\n{_sep}\n\n")
 
         def worker() -> None:
             # run.py calls fix_console_encoding() at import time, which needs
             # the real sys.stdout.buffer. Import it before we redirect stdout.
-            from run import step_fetch, step_link
+            try:
+                from run import step_fetch, step_link
+            except Exception as _import_err:
+                self.after(0, lambda: self.post_log(
+                    f"\n✗  Failed to load pipeline: {_import_err}\n"
+                ))
+                self.after(0, self._pipeline_done)
+                return
+
             self._redirect_output()
             try:
                 from arma_modlist_tools.parser  import parse_modlist_html