feat: add sensitive-data redaction before translation requests

[adinschmidt]

Summary

• Adds a privacy layer that detects and redacts sensitive data (phone numbers, US SSNs, Canadian SINs, email addresses) before text is sent to any translation provider API
• Blocks translation entirely when the user's selection overlaps browser password fields or sensitive form inputs (e.g., autocomplete="current-password", cc-number)
• New src/shared/sensitive.ts module with regex-based pattern detection and HTML-aware redaction that preserves tag structure
• User-facing "Redact sensitive data" toggle in the options page, on by default
• i18n strings added for all 12 supported locales

How it works

1. Regex patterns match phone numbers, SSNs (with IRS-rule exclusions), Canadian SINs, and emails
2. redactSensitiveHTML() splits HTML into tag/text segments, applies patterns only to text nodes, preserves all markup
3. Redaction is applied at the 3 orchestration functions in background.ts that feed all API call paths — no translation request can bypass it
4. In content.ts, the translate button is hidden and extractSelectedHtml() returns null when the selection touches a sensitive form field
5. Matches are replaced with typed markers like [REDACTED:PHONE] so the LLM knows content was intentionally removed

Test plan

• bun run build succeeds with no errors
• Load extension → Options → verify "Redact sensitive data" toggle is visible and defaults to ON
• Select text containing a phone number (e.g., "Call 555-123-4567") → translate → verify console shows redaction log and API receives [REDACTED:PHONE]
• Select text with SSN format (e.g., "SSN: 123-45-6789") → verify it's redacted as [REDACTED:SSN]
• Select text with email → verify redacted as [REDACTED:EMAIL]
• Navigate to a login page → select text from a password input → verify the translate button does NOT appear
• Full-page translate on a page with phone numbers inside <b> tags → verify HTML tags preserved and numbers redacted
• Toggle redaction OFF in settings → verify sensitive data passes through unchanged

Closes #28