fix(print): keep PDF exports readable in all themes

[PathGao]

Summary

PDF exports now use a paper layout of their own instead of inheriting the active screen theme, and diagrams are rebuilt for paper rather than recoloured.

Paper layout. The page gets its margin from the document body so a dark theme no longer leaves the sheet edges unpainted. Wide tables distribute columns from their content and wrap at word boundaries, alerts and table rows stay inside page boundaries, and the properties panel no longer overflows.

Diagrams. Mermaid bakes the active theme into the SVG it emits — fills, strokes and label colours are attributes on the generated elements, not variables a stylesheet can retarget. My first attempt overrode them from the print stylesheet, which means enumerating Mermaid's internal class names; those differ per diagram type and drift between releases. Flowchart nodes sit under g.node, sequence actors are a bare rect.actor, and flowchart labels moved into foreignObject in v11 while sequence labels are still <text>.

Missing one is worse than no override at all: the label rule lands, the fill rule does not, and dark text ends up painted on a near-black box. Measured against Mermaid 11.12 rendered with the dark theme, the enumerated rules left sequence diagrams at a contrast ratio of 1.18 — effectively invisible — while flowcharts reached 4.94.

So this now rebuilds the diagrams instead. The screen render keeps the diagram source on its container, and an export re-renders every diagram with Mermaid's light theme before printing, restoring the on-screen rendering afterwards. The same measurement gives 4.75 for sequence diagrams and leaves flowcharts unchanged. It stays correct for diagram types this version has never seen, and the 36 lines of recolouring CSS are gone. Diagram pagination rules stay.

Fixes #232

Validation

• npm run check — 0 errors, 0 warnings
• npm test — 161/161, including a new scripts/mermaidPrintTheme.test.ts covering the theme decision, source round-trip, re-render, idempotent restore, and the failure path where a diagram that cannot be rebuilt keeps its screen rendering
• Contrast figures above were measured in Chromium against Mermaid 11.12.2 for flowchart, sequence and gantt diagrams, before and after the change
• Page geometry checked across a 111-page export: top margin 54–60pt against a 0.75in target, no page below 40pt