* refactor(frontend): extract placeholder detection utility with unit tests
* fix(frontend): pass prompt text directly to onSelectPlaceholder to avoid stale DOM read
The onSelectPlaceholder callback was reading textarea.value immediately
after textInput.setInput(prompt), but React state updates are async so
the DOM had not yet reflected the new value. This caused the placeholder
auto-selection to silently fail when the textarea was previously empty.
Fix: accept the new text as a parameter instead of reading from the DOM.
* fix(frontend): resolve duplicate findSuggestionTemplatePlaceholder identifier
After rebasing onto main, the function existed both inline in
input-box-helpers.ts (from #3764) and as an import from our new
placeholders module, causing TS2300 duplicate identifier errors.
- Remove duplicate import in input-box.tsx
- Replace inline function in input-box-helpers with re-export from
@/core/suggestions/placeholders
- Export SUGGESTION_TEMPLATE_PLACEHOLDER_PATTERN from placeholders module
* refactor(frontend): remove dead hasUnreplacedPlaceholder export
No production call site uses this boolean wrapper — both existing
checks need the {start,end} range from findSuggestionTemplatePlaceholder.
Drop the function and its two unit tests.
