fix(dashboard): validate dist exists when --skip-build is set

Follow-up to PR #23824. Adds two correctness fixes on top of the
contributor's salvaged commit:

1. Stale-dist fallback no longer gated on `fatal=False`. `cmd_dashboard`
   passes `fatal=True` and is the primary scenario this fallback is for
   (issue #23817 — Windows Scheduled Task at logon). The previous gate
   meant the fallback never fired in the case it was designed for.

2. `--skip-build` now verifies the dist actually exists before starting
   the server. Without this, a misconfigured pre-build would launch the
   dashboard pointing at a missing dist and silently serve 404s. We now
   exit 1 with a clear "pre-build first: cd web && npm run build"
   message, and on success print which dist directory is being used.

Verified end-to-end on Linux:
- build fails + stale dist (fatal=True)  -> fallback fires
- build fails + no dist (fatal=True)     -> exit 1 with stderr surfaced
- build fails + stale dist (fatal=False) -> fallback fires
- --skip-build + missing dist            -> exit 1 with clear guidance
- --skip-build + valid dist              -> 'Skipping web UI build...'

hermes_cli/main.py

@@ -9120,6 +9120,21 @@ def cmd_dashboard(args):
     if "HERMES_WEB_DIST" not in os.environ and not getattr(args, "skip_build", False):
         if not _build_web_ui(PROJECT_ROOT / "web", fatal=True):
             sys.exit(1)
+    elif getattr(args, "skip_build", False):
+        # --skip-build trusts the caller to have pre-built the web UI.
+        # Verify the dist actually exists; otherwise the server will start
+        # and serve 404s with no obvious cause (issue #23817).
+        _dist_root = (
+            Path(os.environ["HERMES_WEB_DIST"])
+            if "HERMES_WEB_DIST" in os.environ
+            else PROJECT_ROOT / "hermes_cli" / "web_dist"
+        )
+        if not (_dist_root / "index.html").exists():
+            print(f"✗ --skip-build was passed but no web dist found at: {_dist_root}")
+            print("  Pre-build first:  cd web && npm install && npm run build")
+            print("  Or drop --skip-build to build automatically.")
+            sys.exit(1)
+        print(f"→ Skipping web UI build (--skip-build); using dist at {_dist_root}")
 
     from hermes_cli.web_server import start_server