Fix #1007: don't export empty audio folders; trim + gate the warning modal

[sampjvv]

Closes #1007.

Problem

On an audio export that includes a file/chapter with no audio: the wizard wrote an empty per-book folder containing only a NOTICE.txt, and the step-2 "Files Without Audio" warning could be dismissed (X / click-outside) without acknowledging it.

Testing also surfaced an inconsistency: a book is one .codex file and chapters are milestones within it. The warning/NOTICE logic was per-file, but the exporter silently drops per-chapter — so an empty chapter inside an audio-bearing book vanished with no feedback at all.

Changes

1. Trim the warning modal (projectExportView.ts) — drop the now-inaccurate "…their exported audio folders will be empty" / "…will produce empty output…" wording from the shared warning popup; keep the header + "The following files have no audio/text translations." + file list.

2. Stop writing the empty folder + NOTICE.txt (exportHandler.ts) — delete generateMissingContentNotices() (the only code creating the per-book folder + NOTICE.txt), its call site, and the now-unused analyzeNotebookContent import. reporter.complete() is unchanged.

3. Report skips in the completion summary instead — file and chapter level (audioExporter.ts) — per book, track which selected milestones produced audio. A whole no-audio book → "Skipped (no audio): N book(s) — …"; selected-but-empty chapters → "Chapters skipped (no audio): GEN 1; …" (book-prefixed, capped at 6). These flow through the existing extraMessages → completion summary, so there's no new plumbing. Only chapters that were selected but produced no audio task count (download failures stay in their existing category; the whole-book opt-out is excluded).

4. Require explicit OK to dismiss the warning (projectExportView.ts) — the warning is a full-screen blocking modal; removing the X button and the click-outside dismissal leaves OK as the only way out, so the export can't proceed until it's acknowledged. Applies to both the audio and text warnings (shared popup). No keyboard/Escape bypass exists for this popup.

Verification

• tsc --noEmit + eslint + npm run compile clean.
• New tests (src/test/suite/export/audioSkipSummary.test.ts): book-level (no NOTICE.txt written + skip line in the summary) and chapter-level (an empty selected chapter is reported as skipped, not silently dropped). Existing exportCancellation test still passes — 3 export tests green.
• Manual: (a) mixed text-only + text+audio export → trimmed modal, no NOTICE.txt, skipped books/chapters listed in the summary; (b) the warning modal has no X and ignores click-outside — only OK dismisses it.

Note: the warning-modal changes are in the extension's wizard HTML string (built by npm run compile), which has no DOM test harness in the suite — verified manually.

🤖 Generated with Claude Code

[sampjvv]

Follow-up edge cases — "Files Without Audio" warning (out of scope for this PR)

Flagging for @LeviXIII to look into. The no-audio warning only fires the moment you click the Include Audio option, so it can be bypassed when the selection changes afterward. Two cases it misses:

1. Deselecting the audio chapters → no warning. Export a book that has audio (no warning, correct). Then on the chapter-selection step, deselect the only chapter(s) that actually contain audio. The remaining selection has no audio, but you get no warning — you just export "0 audio files." → Should warn, on the chapter step, when the current selection contains no audio at all.

2. Re-selecting files after the format is set → no warning. Pick a book with audio, choose Include Audio (warning logic runs, nothing to flag), advance. Go back and add a no-audio book to the selection. Because the audio format is already remembered, you can advance straight through with no warning about the no-audio book. → Should re-check (and warn) whenever the file selection changes, not just on the audio-option click.

Common thread: the warning is a one-shot trigger; it should re-evaluate whenever the file or chapter selection changes (i.e. gate on "current selection has no audio," re-checked at each step).

[sampjvv]

Update: scenario 2 (re-selecting files after the audio format is already set) is now fixed in this PR (53c7aba) — arriving at step 2 from the file step re-runs the no-audio check when the audio option is still selected and the file selection changed. An unchanged, already-acknowledged selection is not re-warned. Scenario 1 (chapter-level deselection) remains open.