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/popup/popup.js

@@ -713,7 +713,7 @@ function renderTestDiff() {
 
   const smartResult = SmartPatterns.substitute(input, identity);
   const explicitResult = SubstitutionEngine.substitute(smartResult.text, mappings);
-  const secretResult = SecretScanner.redact(explicitResult.text);
+  const secretResult = SecretScanner.redact(explicitResult.text, settings.customSecretPatterns);
 
   const allReplacements = [
     ...smartResult.replacements,