feat: secret scanner — auto-detects and redacts API keys, tokens, credentials

Adds a secret scanning layer that runs after identity/explicit
substitutions. Catches secrets the user didn't configure:

- API keys: OpenAI (sk-), Anthropic (sk-ant-), Google (AIza),
  AWS (AKIA), GitHub (ghp_), GitLab (glpat-), Slack (xox),
  Stripe (sk_live/test), SendGrid (SG.)
- Auth: Bearer tokens, key=value assignments (password=, secret=,
  api_key=, token=), private key blocks (-----BEGIN PRIVATE KEY-----)
- Connection strings: mongodb://, postgres://, mysql:// with creds
- PII: SSN (xxx-xx-xxxx), credit card numbers (Visa/MC/Amex/Discover)

Redacted values shown in red in the Test tab. Secrets are truncated
in the activity log (first 8 chars + "...") to avoid logging the
full secret. Enabled by default, toggle in Options.

https://claude.ai/code/session_01Dvgwe7XMoSxnWXkih8p1Cw

src/lib/storage.js

@@ -18,6 +18,7 @@ const DEFAULT_SETTINGS = {
   enabled: true,
   showHighlights: false,
   revealMode: false,
+  secretScanning: true,
   maxLogEntries: 200,
   customDomains: [],
   categories: ['name', 'email', 'phone', 'address', 'ssn', 'dob', 'general'],