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.

CLAUDE.md

@@ -112,6 +112,10 @@ Pass 2 builds `ok_disk_names` — the set of disk names that already match the s
 
 **GUI threading model:** Every network or long-running operation runs in a `threading.Thread(daemon=True)` so the Tkinter event loop is never blocked. The only safe way to update widgets from a background thread is `self.after(0, callback)` — never touch widgets directly from a worker thread. `_poll_log` drains the entire log queue in one `after(80, ...)` tick and does a single batched `CTkTextbox.insert()` call rather than one per log entry, keeping the UI smooth even when `tqdm` emits many rapid updates during downloads. The wizard's "Test Connection" button follows the same pattern: `requests.get` runs in a daemon thread; the result is posted back via `self.after(0, ...)` with widget references captured *before* the thread starts, so stale references cannot update the wrong widgets if the user navigates away mid-request.
 
+**`run_pipeline` worker — import guard:** `from run import step_fetch, step_link` is performed inside its own `try/except` *before* stdout is redirected. If this import fails for any reason the exception is posted to the log via `self.after(0, ...)` and `_pipeline_done` is called so the UI resets cleanly. Previously an import failure would silently kill the worker thread and leave the pipeline button disabled forever.
+
+**`build_server_index` progress callback:** Accepts an optional `progress_fn(current, total, name)` callback. `step_fetch` in `run.py` uses this to print `Indexing N/M: @FolderName` every 25 folders so the log never goes silent during the server scan phase. The library itself never calls `print` — the caller owns the I/O.
+
 ### GUI localization (`gui/locales.py`)
 
 All user-facing strings are centralised in `gui/locales.py`. Two languages are supported: English (`"en"`) and Vietnamese (`"vi"`).