refactor: address code-review findings for install detection and diag log

- pwa-install.ts: permanent 'ponomar-ever-installed' marker keeps install
  button hidden on Firefox/Safari after appinstalled clears the offer flag;
  isPwaInstalled moved to core for reuse
- diag-log: self-heal corrupted storage, show MM-DD in tail
- settings: refresh diag panel after clear-cache; uniform completion style
- data-cache.has: plain count(key); document HTML-as-failure intent

src/core/pwa-install.ts

@@ -0,0 +1,46 @@
+/**
+ * PWA installation state detection.
+ *
+ * 'ponomar-ever-installed' is a permanent marker (never removed): set when
+ * the app is installed, launched in standalone mode, or inherited from the
+ * legacy 'ponomar-installed' offer flag. Used to hide the Settings install
+ * button on browsers without getInstalledRelatedApps() (Firefox, Safari).
+ * Distinct from 'ponomar-installed', which only gates the first-launch
+ * offline-offer modal and is cleared on appinstalled for re-offer semantics.
+ */
+
+const EVER_INSTALLED_KEY = 'ponomar-ever-installed';
+
+export function isStandaloneDisplay(): boolean {
+  return (
+    window.matchMedia('(display-mode: standalone)').matches ||
+    (window.navigator as any).standalone === true
+  );
+}
+
+export function markEverInstalled(): void {
+  try {
+    localStorage.setItem(EVER_INSTALLED_KEY, '1');
+  } catch { /* best-effort */ }
+}
+
+export function hasEverInstalledMarker(): boolean {
+  try {
+    return localStorage.getItem(EVER_INSTALLED_KEY) === '1';
+  } catch {
+    return false;
+  }
+}
+
+export async function isPwaInstalled(): Promise<boolean> {
+  if (isStandaloneDisplay()) return true;
+  if (hasEverInstalledMarker()) return true;
+  try {
+    const navAny = navigator as any;
+    if (typeof navAny.getInstalledRelatedApps === 'function') {
+      const apps = await navAny.getInstalledRelatedApps();
+      if (Array.isArray(apps) && apps.length > 0) return true;
+    }
+  } catch { /* not supported */ }
+  return false;
+}