feat(slides): image beside text, slide numbers, and presenter font scaling

[vpavlin]

Summary

Slides were derived by serialising the document to Markdown and parsing it back:

doc → getHTML() → turndown → Markdown → convertMarkdownToHTML() → HTML → split

Everything Markdown cannot express was silently dropped on the way to the stage. multi-column and font-size extensions already exist in the editor, but no Turndown rule carries them, and md-to-html.ts contains the string column zero times — so an image beside text was impossible on a slide, and author-set font sizes never arrived.

This builds the deck straight from the ProseMirror document instead. Slides remain string[], rendered back through the editor's own schema, so the preview panel, PDF export and share links are untouched.

Three user-facing additions: image beside text, slide numbers, and presenter font scaling.

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Screenshots

1. Image beside text (the headline change)

Before	After

2. Slide numbers and font-size control

4. Windowed vs fullscreen parity

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What changed

package/utils/doc-to-slides.ts (new) — splits the document at structural boundaries (page breaks, headings), then decides breaks by measuring against the real 1080×608 stage rather than counting characters. The old heuristics (7 lines / 1000 chars / 250 words) split slides that visibly had room to spare. Character counting survives as a fallback for environments without layout.

Fitting rules:

• lists paginate between <li> items, with start carried over so ordered numbering stays continuous
• columns and tables are never divided — breaking a side-by-side layout destroys it rather than paginating it
• a heading is never stranded: if what fits is only headings and the next block is indivisible, they stay together and the slide is allowed to run long. A title alone with its content on the next slide is a worse outcome than a slightly tall slide.

presentation-mode.tsx — fullscreen used dangerouslySetInnerHTML while windowed rendered through the editor, so custom nodes could appear in one and not the other. Both now render through the editor. The wrapper is animated via controls rather than remounted, because EditorContent owns the editor's DOM node and re-parenting it per slide is what let the two paths drift apart.

editor.css — windowed presentation type was set at ~2.2% of stage width while fullscreen already used ~1.6%. Aligned to fullscreen's proportions, and --slide-font-scale now threads through both.

ddoc-editor.tsx — Mod+Alt+P opens the deck.

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Bugs found along the way

Not part of the original scope, but worth flagging:

1. Presenting a ddoc could silently edit it. The document editor stays mounted behind the presentation overlay and kept DOM focus, so f, +, -, 0, arrows and space were typed into the document while presenting. Fixed by releasing focus on open and making the presentation editor read-only.
2. Fullscreen ignored font scaling entirely — its 14 viewport-relative sizes never referenced the scale variable.
3. Slide build failures left the loader spinning with nothing logged. Now surfaced.
4. processMarkdownContent in md-to-slides.ts (~200 lines) is exported but never imported, and disagrees with the live path (maxLines 8 vs 7). Left alone here, but it's dead code.

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Behaviour changes reviewers should weigh

• An H1 no longer produces a bare title slide. A heading now opens a slide and content packs beneath it; use an explicit page break for a title-only slide.
• Slides may overflow slightly rather than stranding a heading (see fitting rules above).
• Windowed slide typography is substantially smaller — see screenshot 2.

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Testing

npm run build passes. 11 unit tests added, covering the structural splitter against the real schema via getHeadlessExtensions. Lint unchanged from main (8 pre-existing warnings, 0 errors).

Known coverage gap, stated plainly: the measurement path is unreachable in unit tests — jsdom parses markup but reports every height as 0, so fitSlidesToStage falls back to heuristics there. Everything in that path was verified by hand in the demo app. The rules most worth protecting (columns never split, headings never stranded, lists paginate) regressed at least once during development and would each have been caught instantly by a browser test. A Playwright test asserting real geometry is the natural follow-up.

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Follow-up work, deliberately not in this PR

• Windowed and fullscreen are still two independent stylesheets. This PR aligns their numbers; it does not merge them, so they can drift again — and the PDF path is a third set of rules. Four of the bugs above trace to that split. Defining the slide once in stage-proportional units and having fullscreen scale it would remove the class entirely.
• Font scaling does not re-split the deck. The scale is read when slides are built, so scaling up afterwards can overflow.
• Page numbers are not in the PDF export — handle-print.ts is a separate path with its own stylesheet.
• Images inside columns can under-measure if they have not decoded when the slide is measured.

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