Fix startup crash on empty/missing GitHub state file

- snapshots.py: catch JSONDecodeError on corrupt/empty snapshot files
- server.py: skip writing local file if GitHub content decodes to empty string

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

src/soci/api/server.py

@@ -142,7 +142,10 @@ async def load_state_from_github(data_dir: Path) -> bool:
                 logger.info("No GitHub state file found — starting fresh")
                 return False
             resp.raise_for_status()
-            content = base64.b64decode(resp.json()["content"]).decode("utf-8")
+            content = base64.b64decode(resp.json()["content"]).decode("utf-8").strip()
+            if not content:
+                logger.warning("GitHub state file is empty — starting fresh")
+                return False
             local_path = data_dir / "snapshots" / "autosave.json"
             local_path.parent.mkdir(parents=True, exist_ok=True)
             local_path.write_text(content, encoding="utf-8")

src/soci/persistence/snapshots.py

@@ -57,9 +57,13 @@ async def load_simulation(
         json_name = name or "autosave"
         path = SNAPSHOTS_DIR / f"{json_name}.json"
         if path.exists():
-            with open(path, "r", encoding="utf-8") as f:
-                state = json.load(f)
-            logger.info(f"Loaded snapshot from JSON fallback: {path}")
+            try:
+                with open(path, "r", encoding="utf-8") as f:
+                    state = json.load(f)
+                logger.info(f"Loaded snapshot from JSON fallback: {path}")
+            except (json.JSONDecodeError, ValueError) as e:
+                logger.warning(f"Snapshot file corrupt or empty, ignoring: {path} ({e})")
+                state = None
 
     if not state:
         logger.info(f"No snapshot found: {name or 'latest'} — will start fresh")