feat: custom secret patterns + rename Secret Scanner → Auto Redact

Users can now define custom regex patterns for proprietary token formats,
internal URLs with keys, or any secret the built-in scanner doesn't cover.
Patterns are added/toggled/removed from the Options page and apply to both
the live interception (content.js) and the Test tab (popup.js).

Renamed all user-facing "Secret scanning" labels to "Auto Redact" across
popup and options. Internal variable names (secretScanning, SecretScanner)
kept for backwards compatibility with stored settings.

https://claude.ai/code/session_01KF4i7Ra7zCEDskxDBaNtcT

src/lib/storage.js

@@ -40,6 +40,7 @@ const DEFAULT_SETTINGS = {
   autoAddDetected: true,
   maxLogEntries: 100,
   customDomains: [],
+  customSecretPatterns: [],
   categories: ['name', 'email', 'phone', 'address', 'ssn', 'dob', 'domain', 'password', 'general'],
   browserSync: false,
 };