fix(health): Fix button must not act when the switch was cancelled

Vault health → Fix on entry B while the editor is open and dirty on entry A:
the discard prompt appears, Cancel keeps A open — but openEntryForFix carried
on regardless. It unmasked A's password to cleartext, selected it, and pulsed
the Generate button (the blinking). Wrong entry, and it revealed a password
the user never asked to see.

openSlideOver was void, so callers couldn't tell a cancelled switch from a
successful one. It now returns false when it bails (discard cancelled, entry
not found) and true once the panel actually shows the requested entry;
openEntryForFix checks it before touching the password field.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

js/app.js

@@ -3815,9 +3815,9 @@ async function openSlideOver(id, opts) {
             okText:  'Discard',
             danger:  true,
         });
-        if (!ok) return;
+        if (!ok) return false;   // caller must not act on a panel that didn't switch
     }
-    if (!isNew && !e) return;
+    if (!isNew && !e) return false;
 
     // The editor must be visible when it opens: Settings sits ABOVE the
     // slideover, so a hotkey (Ctrl+Shift+A) fired with Settings open left
@@ -4019,6 +4019,10 @@ async function openSlideOver(id, opts) {
     }
 
     renderGrid();
+    // true = the panel is now showing `id`. Callers that chain an action on
+    // the opened entry (openEntryForFix) MUST check this — a cancelled
+    // discard prompt leaves the previous entry in place.
+    return true;
 }
 
 // "Show history" launcher — placed below the password field for logins,