Harden local backend security boundaries

[FernandoAbishai]

Summary

This PR implements the highest-priority findings from the ScriptCut security audit:

• uses one authenticated backend origin (127.0.0.1) in packaged and Electron development runs
• lets Electron own backend startup during npm run dev, avoiding an unauthenticated duplicate backend
• enables Electron sandbox/web security and validates IPC senders
• removes plaintext credential-storage fallback when OS encryption is unavailable
• narrows CORS to the local Vite origins
• adds configurable upload size limits, restrictive temp-file permissions, and validated HTTP byte ranges
• validates custom AI provider destinations to block cleartext remote targets and private-network SSRF
• replaces unbounded thread creation with a bounded executor and queue backpressure (429)
• removes duplicate backend packaging from the ASAR payload
• adds focused security smoke tests

Security invariants

• privileged local API calls require the per-launch token when launched by Electron
• remote provider endpoints must use HTTPS and may not resolve to private/link-local/reserved networks
• loopback HTTP remains supported for Ollama and 9Router
• background work is limited to a small worker pool and bounded queue
• existing open/save/project dialog APIs remain unchanged

Validation

The branch includes backend/scripts/smoke_security.py, and npm run smoke:backend now discovers all smoke_*.py tests. CI should run lint, frontend build, existing smoke tests, new security regressions, and Python compilation.

Remaining follow-up

This PR intentionally does not yet redesign local file streaming around capability tokens, pin the full Python dependency graph, or migrate the transcription cache. Those are separate changes to avoid mixing architecture and dependency work into the boundary fix.

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