feat(plan): support a dedicated planning model

[luoye520ww]

Summary

• Add an optional Plan-mode model/provider/account slot to Kun settings.
• Route Plan turns to the configured slot while preserving explicit per-turn overrides.
• Expose the setting in the existing Subagent settings editor with English and Chinese labels.

This is the model-routing portion of #878. Project import/export is intentionally separate so it can be reviewed and tested independently.

Compatibility

• An empty Plan slot falls back to the normal conversation model.
• Explicit per-turn model/provider/account values always take precedence.
• Existing agent and chat turns are unchanged.

Tests

npm.cmd exec vitest run src/renderer/src/agent/kun-runtime.test.ts src/renderer/src/components/subagents/SubagentSettingsEditor.test.ts
npm.cmd run typecheck
npm.cmd run lint -- --quiet
npm.cmd run build
git diff --check

Results: 2 test files / 36 tests passed; Typecheck, lint, build, and diff check passed locally. Linux, macOS, and Windows package checks are green in CI.

Issue

Part of #878

[XingYu-Zhong]

Reviewed against current develop after branch update. Scope is focused, the change remains needed, and no blocking issue was found. Required CI must pass before merge.