Surfaces a SingleSelect modal for the terminal theme preference. Bare
\`/theme\` opens the picker; \`/theme <name>\` keeps its existing
"persist and report" behaviour. Moves the slash entry from the
advanced group to setup and adds high-contrast to the arg completer.

Adapted from J3y0r's PR — repointed onto current main (which had
since reformatted App.tsx around the InflightProvider wrap from
#566) and the count test in tests/ui-slash-suggestions.test.tsx
updated from 12 → 11 advanced commands now that /theme has moved
out of the advanced bucket.

Thanks @J3y0r for the contribution.

…ht set (esengine#566)

Closes esengine#557

Tool spinners occasionally kept spinning after the underlying work had
finished. The cause: status was set imperatively from paired tool_start
/ tool events, and every code path that exits a dispatch had to
remember to emit the closing event — normal return, throw, abort,
storm-breaker, network drop. Miss one and the card was stuck.

Switch the running flag to a derived view:

- New InflightSet (src/core/inflight.ts) with add / delete / has /
  subscribe + notify-on-mutation. Parallel finally-driven cleanup is
  the contract: the language guarantees finally on every exit path,
  so there's no closing event to forget.
- CacheFirstLoop holds one. runOneToolCall adds the call's id at
  function entry and deletes it in finally; the chunk-loop pre-adds
  before yielding tool_start so the very first card render already
  reflects the running state. clearLog drains the set on /new.
- LoopEvent.tool_start / tool now carry callId — same id used as the
  inflight key. TurnTranslator threads it into Scrollback.startTool,
  which uses it as the card id so the card and the inflight key match.
- InflightProvider + useIsInflight (src/cli/ui/state/inflight-context.tsx)
  expose the set as a React context backed by useSyncExternalStore.
  ToolCard's toolStatus consults the hook for "running"; card.done is
  no longer the spinner discriminator, only terminal flavor (ok /
  rejected / aborted / error).

Tests:
- tests/inflight.test.ts — InflightSet contract: idempotent add,
  notify-only-on-change, listener errors don't break the gate, clear,
  finally-on-throw, finally-on-abort, parallel allSettled.
- tests/loop-inflight.test.ts — runOneToolCall finally clears the
  set on success and on tool throw; tool_start carries callId
  matching the inflight key; clearLog drains.

Out of scope (follow-up): subagent runId in the same set; deriving
streaming/reasoning running flags. Both are mentioned in the issue
but live behind different leak surfaces; the tool-card path is the
one users see most.

Surfaces a SingleSelect modal for the terminal theme preference. Bare
\`/theme\` opens the picker; \`/theme <name>\` keeps its existing
"persist and report" behaviour. Moves the slash entry from the
advanced group to setup and adds high-contrast to the arg completer.

Adapted from J3y0r's PR — repointed onto current main (which had
since reformatted App.tsx around the InflightProvider wrap from
esengine#566) and the count test in tests/ui-slash-suggestions.test.tsx
updated from 12 → 11 advanced commands now that /theme has moved
out of the advanced bucket.

Thanks @J3y0r for the contribution.