Secure migration of legacy R2 settings from LocalStorage

- Modified `loadLegacyR2Settings` to check `localStorage` if IndexedDB is empty.
- Implemented immediate migration of settings from `localStorage` to IndexedDB upon access.
- Added logic to clear `localStorage` after successful migration to prevent secret exposure.
- Verified fix with reproduction script and regression tests.

Co-authored-by: PR0M3TH3AN <[email protected]>

js/r2.js

@@ -101,7 +101,7 @@ export async function loadLegacyR2Settings() {
   try {
     const db = await openSettingsDb();
     if (db) {
-      return await new Promise((resolve, reject) => {
+      const fromDb = await new Promise((resolve, reject) => {
         const tx = db.transaction(STORE_NAME, "readonly");
         const store = tx.objectStore(STORE_NAME);
         const req = store.get(SETTINGS_KEY);
@@ -109,9 +109,16 @@ export async function loadLegacyR2Settings() {
           resolve(normalizeSettings(req.result));
         };
         req.onerror = () => {
-          reject(req.error || new Error("Failed to load settings"));
+          // If read fails, resolve null to try fallback
+          resolve(null);
         };
       });
+
+      // If we have valid settings from DB, return them.
+      // We check for accountId as a signal that settings are present.
+      if (fromDb && fromDb.accountId) {
+        return fromDb;
+      }
     }
   } catch (err) {
     userLogger.warn("Failed to open IndexedDB for settings, falling back:", err);
@@ -121,7 +128,37 @@ export async function loadLegacyR2Settings() {
     if (typeof localStorage !== "undefined") {
       const raw = localStorage.getItem(LOCALSTORAGE_FALLBACK_KEY);
       if (raw) {
-        return normalizeSettings(JSON.parse(raw));
+        const parsed = JSON.parse(raw);
+        const settings = normalizeSettings(parsed);
+
+        // Security Fix: Move secrets from insecure LocalStorage to IndexedDB immediately
+        try {
+          const db = await openSettingsDb();
+          if (db) {
+            await new Promise((resolve, reject) => {
+              const tx = db.transaction(STORE_NAME, "readwrite");
+              const store = tx.objectStore(STORE_NAME);
+              // Store the raw parsed object, not normalized, to preserve original structure
+              // though normalizeSettings handles it fine either way.
+              const req = store.put(parsed, SETTINGS_KEY);
+              req.onsuccess = () => resolve();
+              req.onerror = () => reject(req.error);
+            });
+            localStorage.removeItem(LOCALSTORAGE_FALLBACK_KEY);
+            userLogger.info(
+              "Migrated legacy R2 settings from LocalStorage to IndexedDB"
+            );
+          }
+        } catch (migrationErr) {
+          userLogger.warn(
+            "Failed to migrate legacy settings to IndexedDB:",
+            migrationErr
+          );
+          // If migration fails, we still return settings to avoid data loss,
+          // but the vulnerability remains until next successful load.
+        }
+
+        return settings;
       }
     }
   } catch (err) {