fix: handle bare {"stage":"name"} loading payload; harden watcher against parser errors

The stage-transition payload {"state":"loading","payload":{"stage":"mmproj_model"}}
lacks the stages/current/value keys — accessing them raised KeyError which crashed
the LogWatcher task, freezing the UI at the last good state indefinitely.

- log_parser: skip loading payloads missing stages/current/value (next message
  has the full payload within milliseconds)
- log_watcher: wrap on_line calls in try/except so any future parser error
  keeps the tail loop alive instead of killing the task

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

log_parser.py

@@ -200,6 +200,10 @@ class LogParser:
         payload = data.get("payload")
 
         if s == "loading" and payload:
+            # Some transition messages only carry {"stage":"name"} with no
+            # progress info — skip them; the next message has the full payload.
+            if not all(k in payload for k in ("stages", "current", "value")):
+                return None
             loading = LoadingInfo(
                 stages=payload["stages"],
                 current=payload["current"],