Show attachment counts in the compact composer

[bb-slop-cop]

Human comments

What was wrong

After selecting files on mobile, focus loss collapses the follow-up composer. Compact layout previously omitted all attachment UI, so successful uploads appeared to have failed until the editor was focused again (#3177).

What changed

Compact composers now show a paperclip and the total attachment count beside the draft text in the existing single row. The indicator has an accessible singular/plural label and no removal button. Focusing the composer exposes the existing image previews and file removal controls. Removing a focused attachment explicitly blurs its button before unmount so the composer can settle back to compact layout.

This replaces the original PR's forced-expansion approach: attachments remain visible as a count without keeping the editor expanded. No SDK, CLI, stored-data, or wire contract changes; no host-daemon protocol bump is required.

How you verified

• Reproduced the original missing-preview behavior on pristine main using sawyerhood/dev-browser at a 390×844 touch viewport and a real Chromium file chooser.
• Regression coverage checks mixed image/file counts, singular count, no compact removal controls, expanded removal controls, and an empty attachment list.
• Ran the focused and adjacent promptbox suite through Turbo: 388 passed, 3 skipped.
• Turbo app typecheck, build, and lint passed; lint has existing warnings and no errors. Formatting and git diff --check passed.
• Live count-only QA on the optimized app launched through pnpm start:worktree: 50px compact row with draft text, accessible “2 attachments” indicator, zero compact removal controls, expansion for removal, count update to one, and clearing the count after final removal.
• Earlier iterations exercised actual iOS Simulator Safari picker return and removal. The final count-only design was verified in Chromium touch emulation, not rerun in Safari or TestFlight.

Fixes #3177

AGENT GENERATED

[dillonzq]

I independently verified this fix on the latest main (682f0d2c5dc5bbe920d4a253f66a6c5cedcb955b) and would appreciate merging it soon.

• Before the fix, the regression test consistently failed when an attachment arrived after the mobile composer lost focus: the composer remained compact instead of expanding.
• After applying this PR, the regression passes; removing the final attachment also restores the normal compact state.
• The attachment removal component tests pass, confirming that the removal callback and local preview cleanup still work.
• The complete @bb/app suite passes: 496 test files, 4,095 tests passed, 4 skipped.
• Typecheck, formatting, and git diff --check pass; lint reports 0 errors.

This addresses both symptoms: the composer collapsing after an image upload and the attachment controls being unmounted before the image can be removed.