feat(json-chat): add 'send on Enter' setting; show active send hotkey on button + placeholder

[frenchie4111]

Summary

• Adds a new jsonModeSendOnEnter setting (default off) so users can opt into the conventional plain-Enter-to-send behavior in the JSON-mode chat composer. When off, the historical Cmd/Ctrl+Enter behavior is preserved. Shift+Enter is honored as the newline shortcut in both modes (just falls through to native textarea behavior).
• Plumbs the setting through the full 5-file slice checklist: shared slice + test, persistence, initial state, IPC handler in main, backend method, and ElectronAPI type.
• Composer placeholder and Send button now show the active hotkey — ⌘↵ / ↵ on Mac, Ctrl+Enter / Enter elsewhere — and the button carries a screen-reader-friendly aria-label spelling it out (Send (Cmd+Enter), etc.).
• IME composition guard added (e.nativeEvent.isComposing + keyCode === 229), so Enter during CJK input never sends.
• The @-mention picker's Enter handler is untouched — it still consumes plain Enter while the popover is open.
• New toggle in Settings → JSON-mode card (right under Chat density), labelled "Send messages with Enter" with subtext "Plain Enter sends; Shift+Enter inserts a newline. When off, Cmd/Ctrl+Enter sends."

Test plan

• Default (off): Cmd/Ctrl+Enter sends; plain Enter inserts a newline; Send button shows Send ⌘↵ / Send Ctrl+Enter; placeholder reads Message Claude — Cmd+Enter to send.
• Toggle on in Settings: plain Enter sends; Shift+Enter inserts a newline; Send button shows Send ↵ / Send Enter; placeholder reads Message Claude — Enter to send, Shift+Enter for newline.
• IME guard: switch input source to Japanese/Korean, type composing chars + Enter → no send.
• @-mention picker still works: type @ to open it, plain Enter still picks the suggestion (doesn't send the message), regardless of the setting.
• npm run typecheck, npx electron-vite build, and npx vitest run src/shared all green.

🤖 Generated with Claude Code