feat(remote): add ssh target picker UI

[luoye520ww]

Summary

• Adds the renderer/main SSH remote target picker UI on top of the remote target core branch.
• Lets a thread pick, test, persist, and clear remote SSH target bindings without mixing the backend core diff into this review.
• Includes the composer slot integration and renderer runtime bridge updates needed by the picker.

Tests

• npm.cmd test -- src/renderer/src/agent/kun-runtime.test.ts src/main/ipc/app-ipc-schemas.test.ts src/renderer/src/store/chat-store-thread-actions.test.ts
• npm.cmd --prefix kun run typecheck
• npm.cmd --prefix kun run build
• git diff --check codex/p0-remote-target-core...HEAD

Stacking

• This PR is stacked on codex/p0-remote-target-core, which corresponds to feat(remote): add guarded ssh target runtime core KunAgent/Kun#811. After feat(remote): add guarded ssh target runtime core KunAgent/Kun#811 merges, this branch should be rebased/retargeted to KunAgent/Kun:develop as the official PR.