fix(system): flag changes made while System was closed and keep embedded galleries fixed

Final review fixes for System S3 (Appearance):

- I1: the on-demand metadata deviation is D124 (TESTING.md, inspector
  markers); TESTING.md names the spec that pins it, since no concept
  comparison renders that state.
- I2: System checks the display defaults for conflicts on mount, not only
  on storage events, and a browser write refuses a draft another page
  overtook, so a draft is never saved over another tab's value without
  the conflict status first. New two-tab spec.
- I3: the layout and size defaults start the Images page only; embedded
  galleries (a prompt history's Shared gallery and dialogs) keep
  Grid/Comfortable.
- M1: host rows whose /settings read failed say "Server settings could
  not be read."
- M2: a rejected timeout shows the "Some changes were not saved" status.
- M3: the browser loads on mount and on storage events never reject
  unhandled.
- M4: the next accepted write stores the tab-only changes and clears the
  overlay.
- M5: cache off never re-reads a source-changed selection, and refresh()
  never loads a deferred one.
- M6: the System fixture refuses a config.json outside its temp root and
  restores it with os.replace.
- M7: the inspection read counts wait on a request barrier, not frames.
- M8 recorded as an open saved-views question in TESTING.md.

tools/vendor/frontend-coverage-baseline.json

@@ -469,15 +469,15 @@
   },
   "workspace/system-state.mjs": {
     "lines": 98,
-    "branches": 93,
+    "branches": 94,
     "functions": 98,
     "statements": 98
   },
   "workspace/system.mjs": {
-    "lines": 96,
+    "lines": 98,
     "branches": 92,
-    "functions": 84,
-    "statements": 96
+    "functions": 85,
+    "statements": 98
   },
   "workspace/theme.mjs": {
     "lines": 98,