Add a localhost credentials manager

[kwannoel]

Why

The Controller needs a local credentials manager so users can store and use sensitive values such as API keys, tokens, and service credentials without hardcoding them into project files, shell history, or per-session prompts.

What

Add a credentials manager that runs on localhost and can be used by the app and managed terminal sessions.

A first version should support:

• Creating, updating, listing, and deleting named credentials locally
• Keeping credential values out of normal logs, terminal transcripts, and frontend state dumps
• Supplying credentials to launched sessions only through an explicit, auditable path
• Persisting credentials across app restarts on the local machine
• Clear UI/API behavior for missing, renamed, or revoked credentials

Constraints

• Must run locally; do not depend on a hosted secrets service for the core path
• Bind only to localhost by default
• Avoid exposing raw secret values once saved unless the user explicitly reveals or rotates them
• Follow the app Axum + Svelte conventions
• Consider macOS Keychain or another OS-backed store if it fits the local-first model

Acceptance Criteria

• Users can manage named credentials from the local app
• Managed Claude Code sessions can opt into specific credentials without leaking unrelated secrets
• Secrets are redacted in logs, API responses, and terminal-visible diagnostics
• Restarting the app does not lose saved credentials
• Tests cover the redaction and credential access boundaries

Related

• add secret backup mechanism #72 covers backup/export/import of secrets and assumes a secret store exists. This issue is for the localhost credentials manager itself.