feat: add MiniMax M2.7 as configurable backend for Ask about Code

[octo-patch]

Summary

• Adds MiniMax M2.7 (204K context) as an alternative LLM provider for the Ask about Code inline Q&A feature
• Users can switch between Claude Code (default) and MiniMax in the Settings → Ask about Code section
• When MiniMax is selected, the app calls the MiniMax API directly via its OpenAI-compatible endpoint — no claude CLI required
• API key is stored in app state and persisted across restarts

Changes

File	What changed
electron/ipc/ask-code-minimax.ts	New MiniMax streaming backend: SSE parser, abort-signal handling, Bearer auth
electron/ipc/ask-code-minimax.test.ts	12 unit tests (chunks, errors, cancel, auth header, model, temperature, stream flag)
electron/ipc/ask-code.ts	Route to MiniMax or Claude based on provider arg
electron/ipc/register.ts	Forward provider / minimaxApiKey from IPC call args
src/store/{types,core,ui,store,persistence}.ts	New askCodeProvider + minimaxApiKey store fields, persisted to state.json
src/components/SettingsDialog.tsx	Provider dropdown + conditional API-key input
src/components/AskCodeCard.tsx	Pass provider + apiKey through IPC
README.md	Mention MiniMax as a supported Ask-about-Code provider

Test plan

• Run npm test — 12 new passing tests, all existing tests still pass
• Open Settings (Ctrl+,) → see new "Ask about Code" section with provider dropdown
• Select MiniMax, enter a valid MINIMAX_API_KEY, select code in a task, ask a question — response streams in
• Switch back to Claude Code — Claude CLI is used as before
• Restart the app — provider and key are persisted