fix(admin): address Copilot review findings

- Reflect built-in defaults for boolean settings (ALLOW_PRIVATE_URLS
  defaults on) and allow clearing a saved boolean back to default,
  so the SSRF toggle matches actual runtime behavior.
- Harden JSON loading: filter settings values to strings only, and
  schema-validate stored ADMIN_PROVIDERS entries, dropping malformed
  ones instead of letting them reach runtime code.
- Set beforeunload returnValue so the unsaved-changes prompt shows in
  all browsers; reject non-finite numbers in settings validation.
- Fix README/CN/JA docs that claimed the panel auto-generates
  AI_MODELS_CONFIG (providers are merged at read time, not written).
- Add unit tests for corrupted-file value filtering and provider
  schema validation.

lib/admin/settings.ts

@@ -29,8 +29,18 @@ export function loadSettings(): Record<string, string> {
     try {
         const raw = fs.readFileSync(getSettingsPath(), "utf8")
         const parsed = JSON.parse(raw) as SettingsFile
-        cachedSettings =
-            parsed && typeof parsed.values === "object" ? parsed.values : {}
+        // Keep only string values — a hand-edited or corrupted file could
+        // hold null/arrays/numbers that would otherwise be overlaid onto
+        // process.env and coerce to junk like "[object Object]".
+        const values: Record<string, string> = {}
+        const raw_values =
+            parsed && typeof parsed.values === "object" && parsed.values
+                ? parsed.values
+                : {}
+        for (const [key, value] of Object.entries(raw_values)) {
+            if (typeof value === "string") values[key] = value
+        }
+        cachedSettings = values
     } catch (err: any) {
         if (err?.code !== "ENOENT") {
             console.error("[admin-settings] Failed to read settings file:", err)