Harden Email Translator input guards

[Chenxizhe2025]

Summary

Closes #497.

Adds a folder-local security and performance hardening surface for the isolated Email Translator tool. The change stays entirely inside tools/v2/individual/email-translator/ and does not wire the tool into the main app.

Changes

• Added services/email-translator-guards.mjs with request normalization, language checks, unsafe markup detection, secret-looking content blocking, bounded metadata handling, and translation workload estimation.
• Added deterministic synthetic fixtures covering safe input, tracking-pixel warnings, active markup rejection, secret-looking content, and unsupported language rejection.
• Added Node built-in tests for the local guard contract, large-body clipping, sanitizer behavior, and malformed metadata.
• Added docs/SECURITY_AND_PERFORMANCE.md documenting threat assumptions, unsafe inputs, and performance limits.
• Updated the tool README with the local validation command.

Validation

• node --test tools/v2/individual/email-translator/tests/email-translator-guards.test.mjs - 4 passing
• git diff --check

Boundary check

• Only files under tools/v2/individual/email-translator/ changed.
• No live network calls, secrets, production data, app shell, routing, inbox, wallet, Stellar, database, or shared design system changes.