Add an Enter-to-queue, Cmd/Ctrl+Enter-to-steer follow-up mode

[felipeorlando]

Closes #7

Adds a third queue-steer follow-up mode. While a turn runs: Enter queues, ⌘+Enter (macOS) / Ctrl+Enter steers. Idle sessions send normally on both. Shift+Enter still inserts a newline. The existing Queue-only and Steer-only modes are unchanged.

The keyboard intent flows through the existing submit path — no second queue implementation. Decision logic lives in two pure helpers in the new src/lib/followUp.ts: isSteerShortcut(event, isMac) and resolveFollowUpAction(behavior, steerShortcut). App.tsx resolves the action at submit time; intent === "plan" still forces queue, and explicit followUpBehavior: "steer" callers (queued-row Steer, resume-queue) are unaffected.

Needs careful review

• No other keydown branch was touched. The mention picker, slash/skill picker and compact-command branches still consume Enter first, modifier or not.
• Composition handling is byte-for-byte unchanged. No isComposing guard was added — the upstream IME bug (Composer sends the message when Enter is pressed to commit IME composition (CJK input) hardbeat920/monocode#105) is out of scope, and guarding here would change behavior beyond this issue. Worth exercising with CJK input before merging.
• The Segmented label reads "Shortcut", not the full "Queue, shortcut to steer" — the control is a fixed-width 3-column track and the full phrase wraps. The row description spells out the platform-correct chord.

Verification

npm run check:web — exit 0, 128 files / 1331 tests. tsc --noEmit clean. 7 new tests for the helpers, plus a persistence round-trip test.

No Composer render test — the repo has no React Testing Library setup for it.

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