fix(tui): render composer shape preview at full overlay width

[F0Rextasy]

What

I reviewed the full diff; the Composer Shape preview capped itself at 96 columns so the status bar rendered as a stub, and it now uses the full overlay width like the live editor.

Why

Fixes #12500

Testing

• bun test packages/tui/test/composer-shape-preview.test.ts (4 pass, 1 new: status widths exceed 96 at width 160)
• bun --cwd=packages/tui run check:types (pass)
• oxlint + oxfmt on both changed files (pass)

---

• bun check passes
• Tested locally
• CHANGELOG updated with the required attribution (if user-facing; internal issue fixes use issue links, external contributions add the PR link and contributor credit after creation)

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