fix(web): treat only 2xx as a successful display save

Review feedback on #422.

- showDisplaySaveResult tested `xhr.status >= 400` for failure, so a
  network error — which reports status 0 — was waved through as
  "Display settings saved". That's the same class of false-success bug
  this branch set out to fix. Test the 2xx range instead, and let a
  response body refine a successful verdict without overturning a
  failed one.
- Annotate the _copies_fits_hardware helper, matching the annotated
  helpers already in api_v3.py.
- Cover the vertical divisibility branch: chain_length 2 would divide
  evenly, so only parallel 3 can produce the rejection, which pins the
  branch to the right hardware dimension.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016aUvzTXpEYhNEWYQvFqQU9

web_interface/templates/v3/partials/display.html

@@ -615,12 +615,18 @@ if (typeof window.fixInvalidNumberInputs !== 'function') {
 // `responseJSON` (that's a jQuery property) — read `responseText` and the real
 // status code, otherwise a failed save reports success.
 window.showDisplaySaveResult = function(xhr) {
-    let message = 'Display settings saved';
-    let status = xhr.status >= 400 ? 'error' : 'success';
+    // Only 2xx counts as saved. A network failure reports status 0, which any
+    // `>= 400` test would wave through as success.
+    const httpSuccess = xhr.status >= 200 && xhr.status < 300;
+    let message = httpSuccess
+        ? 'Display settings saved'
+        : 'Display settings were not saved. Check your connection and try again.';
+    let status = httpSuccess ? 'success' : 'error';
     try {
         const data = JSON.parse(xhr.responseText);
         if (data.message) message = data.message;
-        if (data.status) status = data.status;
+        // A body can refine a successful verdict but never overturn a failed one.
+        if (httpSuccess && data.status) status = data.status;
     } catch {
         // Non-JSON body — fall back to the status-code verdict above.
     }