fix: store the LLM API key in SecretStorage, not settings

[MaximeGaudin]

Summary

• slint.llm.apiKey was a plain string setting, so the credential persisted in plaintext settings.json (and in editor sync, or a committed workspace file). The setting is removed.
• The key now lives in vscode.SecretStorage (OS keychain, encrypted at rest), set with slint: Set the model API key (secure storage) (masked input) and removed with slint: Clear the stored model API key. llmArgv/llmEnv read the cached secret; it is still passed only via the SLINT_EDITOR_API_KEY env var, never argv.
• One-time migration on activation: a key already in settings is moved into SecretStorage (most specific scope wins) and every plaintext copy is cleared, with a notification.

Fixes #191

Note: overlaps PR #321 (issue #33), which made the same setting scope: machine. This PR supersedes that part — one of the two package.json changes will need a rebase when they merge.

Test plan

• New regression tests failed before the fix (setting declared; commands absent; migration unimplemented)
• New regression tests pass after the fix (17/17)
• Local checks mirror CI and passed before push (./scripts/check.sh full run)
• CI green on the PR