Make Enter accept the highlighted slash command (fixes picker Enter)

[dannon]

Closes #287.

Opening the slash-command picker and pressing Enter submitted the raw typed text (e.g. the literal /) instead of running the highlighted command -- you had to Tab to complete first, then Enter. This makes Enter behave like Tab+Enter: it completes the highlighted command and runs it, matching standard autocomplete (VS Code palette, shell completion).

What changed

• In the chat input's keydown handler, the open-popup branch now handles Enter: on a highlighted row it calls acceptSlashPopup(...), closes the popup, then submits, instead of falling through to the raw-submit path. acceptSlashPopup dispatches an input event that can transiently re-open the popup for a no-arg command, so we close it again before submit() -- mirroring the existing raw path.
• Pulled the accept-on-Enter decision into a small pure helper (shouldAcceptSlashCommandOnEnter) with unit tests, following the existing input-history-nav pattern.
• Updated the now-stale comments that documented the old "Tab completes / Enter submits raw" design.

Behavior / scope

• Fully-typed command + Enter still runs. When the popup is open it always highlights row 0 (maybeOpenSlashPopup sets it), and a fully-typed command is itself that highlighted row, so accept+submit re-runs the same command (submit() trims the trailing space acceptSlashPopup may add for arg-bearing commands).
• Arg-bearing commands. Per the issue ("select + run"), Enter on a highlighted command runs it; one that needs an argument surfaces its usage, same as submitting a bare command does today. Use Tab to complete-without-running and then type args.
• Shift+Enter still inserts a newline (both Enter paths require !e.shiftKey), so multi-line input and the ArrowUp recalls prompt history instead of moving the cursor up within a multi-line chat input #314 ArrowUp recall behavior are untouched.
• IME composition. An Enter that commits an IME composition now bails early (a single guard at the top of the handler), so it's never treated as a submit on either Enter path.

Testing

• npm test: green (1255 passed); new tests/slash-popup-nav.test.ts pins the helper contract.
• cd app && npx tsc --noEmit: no new errors (renderer files clean; the pre-existing main/preload electron-type errors are unchanged by this diff).
• Not yet live-eyeballed in Orbit. Manual check to run: type /, arrow to a command, press Enter -> the highlighted command should run (previously submitted the raw /); confirm Shift+Enter still inserts a newline and a fully-typed command + Enter still runs.

Note: the pure-helper test covers the accept/skip decision but not the DOM keydown flow itself (the renderer handler isn't reachable from the node test harness) -- the close-ordering, Shift+Enter, and IME behavior are covered by the manual check above.

Reviewed adversarially over two passes; the only flagged correctness issue (Enter during IME composition still hitting the raw submit path) is fixed here.