fix: export generated preview and translate studio UI

[dpakkia]

Summary

Fixes Studio single-frame MP4 export so it renders the generated/edited project preview HTML instead of
falling back to the selected template example.

Also translates remaining hard-coded Chinese strings in the Studio English flow.

Changes

• Prefer project.lastPreviewHtmlPath for single-frame exports when a generated preview exists.
• Fall back to the selected template only when there is no generated preview.
• Translate Studio prompt/card labels, progress messages, confirm buttons, form labels, and related hard-
  coded UI text to English.
• Update format-reply parser tests for the English UI values.

Why

Before this change, a single-frame project could generate a custom preview, but Export MP4 rendered the
selected template source instead. For example, a generated lower-third using the Vignelli template exported
the Vignelli example rather than the generated lower-third HTML.

Verification

Passed:

• ./node_modules/.bin/tsc -p packages/core/tsconfig.json --noEmit
• ./node_modules/.bin/tsc -p packages/cli/tsconfig.json --noEmit
• node --test packages/cli/test/parse-format-reply.test.ts
• Manual Studio export verified: generated lower-third preview HTML exports instead of the Vignelli
  template source.

Notes:

• pnpm typecheck and pnpm lint both failed locally with fetch failed from the pnpm wrapper before
  producing package diagnostics.
• Direct Biome lint on touched files reports existing repo-wide style issues in legacy files, so this PR
  keeps the change focused.

[lefarcen]

Heads-up: PR #55 is also open against the same single-frame export path. Both PRs touch packages/core/src/project.ts, packages/cli/src/studio-server.ts, and packages/project-studio/public/app.js, and both appear to address the generated-preview export mismatch around lastPreviewHtmlPath.

This PR also carries the remaining Studio English UI translation work, while #55 includes the duration/export-timing fix. You and @walliai666 might want to compare approaches so effort doesn't get split unnecessarily — we'll sort out which shape lands from the maintainer side.

[itxaiohanglover]

Good fixes! Simplifying the regex to remove the 、 (ideographic comma) from the narration cleanup is more robust for international content. Translating Chinese UI messages to English improves accessibility. The retry message change is clearer.