fix(viewer): make Full-Width toggle actually toggle width; center default at ~780px

[andrey-golovko]

Summary

The Full-Width toggle in MarkdownViewer.svelte is currently a visual no-op for the document body. The default and .full-width rules are byte-for-byte identical:

/* before */
.markdown-body {
    padding: 50px clamp(24px, 5vw, 50px);
    max-width: 100%;
}
.markdown-body.full-width {
    padding: 50px clamp(24px, 5vw, 50px);   /* same */
    max-width: 100%;                         /* same */
}

The isFullWidth state only feeds the isOverhanging derivation that decides whether the TOC overlaps the content (MarkdownViewer.svelte:183) — it does not affect the body's width or padding.

A side effect is that on wide monitors the body fills the entire viewer pane, producing very long line lengths.

Change

One-line CSS edit — the default .markdown-body now uses a centering padding:

- padding: 50px clamp(24px, 5vw, 50px);
+ padding: 50px max(24px, calc((100% - 780px) / 2));

• On wide screens, (100% - 780px) / 2 produces symmetric gutters that visually center an ~780 px content column. The 780 matches the existing magic number already used by isOverhanging at MarkdownViewer.svelte:183, so TOC overhang behavior is consistent.
• On narrow screens, the max(24px, …) floor keeps a sensible minimum gutter (same 24 px as before).
• .markdown-body.full-width is unchanged — its existing clamp(24px, 5vw, 50px) padding is now a genuine wide alternative, so toggling Full-Width has a visible effect for the first time.

.markdown-body remains the scroll container, so the scrollbar continues to sit at the pane edge (centering via horizontal padding rather than margin: auto).

Out of scope

• No JS/TS changes. isFullWidth state, localStorage persistence, and TOC overhang logic at MarkdownViewer.svelte:180-187 are untouched.
• Content width is hard-coded to 780 px to match the existing constant. Happy to make it a setting in a follow-up if you'd prefer.

Test plan

• npm run build succeeds (Svelte/Vite, no new warnings on touched rules)
• Local Tauri release build (cargo tauri build) succeeds; toggling Full-Width visibly switches between centered ~780 px column and full-width on a wide monitor
• Cross-check at narrow viewport widths (≤ ~830 px) — gutter falls back to 24 px floor as expected
• Cross-check with TOC visible (settings.showToc) — TOC overhang behavior unchanged

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