feat(remote): add target picker UI

[luoye520ww]

Summary

• Adds a renderer Remote Target picker for choosing Local vs SSH Remote before creating a thread.
• Lists SSH hosts from the runtime, tests remote connections, and passes normalized remoteTarget into new thread creation.

Changes

• Adds RemoteTargetPicker UI near the composer/new-session flow.
• Adds renderer runtime helpers for /v1/remote/hosts and /v1/remote/test.
• Adds renderer-side remote target normalization and store plumbing for createThread.

Stacking / dependency

• Depends on feat(remote): wire ssh targets into runtime tools #672 (feat(remote): wire ssh targets into runtime tools).
• This PR targets KunAgent/Kun:develop as requested, so until feat(remote): wire ssh targets into runtime tools #672 merges GitHub will show the feat(remote): wire ssh targets into runtime tools #672 remote runtime/tools diff here too.
• Clean stacked comparison is available at feat(remote): add target picker UI luoye520ww/DeepSeek-GUI#1.

Tests

• npm.cmd test -- src/renderer/src/agent/kun-runtime.test.ts
• npm.cmd run typecheck
• git diff --check

[XingYu-Zhong]

Rebased onto current develop after remote core merge and verified renderer runtime, IPC schema tests, and typecheck.