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/locales.py

@@ -26,6 +26,7 @@ _EN: dict[str, str] = {
     "nav.settings":                  "Settings",
 
     # ── Pipeline step headers (printed to log) ───────────────────────────────
+    "pipeline.starting":             "Pipeline started",
     "pipeline.step1_name":           "Parse presets",
     "pipeline.step2_name":           "Compare presets",
     "pipeline.step3_name":           "Download mods",
@@ -266,6 +267,7 @@ _VI: dict[str, str] = {
     "nav.settings":                  "Cài đặt",
 
     # ── Pipeline step headers ────────────────────────────────────────────────
+    "pipeline.starting":             "Pipeline đã bắt đầu",
     "pipeline.step1_name":           "Phân tích preset",
     "pipeline.step2_name":           "So sánh preset",
     "pipeline.step3_name":           "Tải mod",