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.html

@@ -198,8 +198,8 @@
     <section class="tab-content" id="tab-options">
       <div class="setting-item">
         <div class="setting-label">
-          <strong>Secret scanning</strong>
-          <span class="setting-desc">Auto-redact API keys, tokens, passwords, SSNs, credit cards</span>
+          <strong>Auto Redact</strong>
+          <span class="setting-desc">Automatically redact API keys, tokens, passwords, SSNs, credit cards, and custom patterns</span>
         </div>
         <label class="toggle"><input type="checkbox" id="optSecretScanning" checked><span class="toggle-slider"></span></label>
       </div>