[gnacho]

Closes #153.
Closes #156.

What changed

Named page templates (#153). A template groups margins, body and monospace typography, line height, heading scale and spacing, colour scheme, header and footer, and a title block built from the document title. Four built-ins ship: Article (today's defaults), Manuscript, Letter and Compact. Custom keeps the granular controls exactly as they were.

The selector lives in Preferences > Export and the choice persists in GSettings (export-template). The granular rows are insensitive unless Custom is selected, because the template would override them anyway. Page size and orientation stay independent: they describe the paper, not the layout.

Print preview (#153). A new "Print Preview…" menu item next to "Export to PDF…" runs the same GtkPrintOperation with PrintOperationAction::Preview, so the preview is drawn by the export pipeline itself and is WYSIWYG by construction.

User stylesheet (#156). A CSS file can be loaded in Preferences > Export and is applied on top of the selected template. It is a mapped subset, not a CSS engine: @page margins, and on body/html/:root, p, h1-h4, pre/code, blockquote, a and table the properties font-family, font-size, line-height, color, background-color, margin-bottom and the custom property --block-spacing. Unknown selectors and properties are ignored with a one-line warning on stderr, never aborting the export. Removing the file restores the template output byte for byte.

Notes

• No new dependencies. The layout is still src/export_pdf.rs and its pagination rules; the CSS parser is a new src/export_css.rs with no GTK in it.
• Custom reproduces the previous output exactly: same margins (paper defaults), 1.4 line height, 10 pt block spacing, current heading scale, no header, footer or title block.
• Header and footer are drawn inside the page margins at 8 pt in the muted colour, so they do not eat into the text area.
• Page backgrounds, including those set from CSS, now cover the whole sheet rather than only the printable area.

Testing

• xvfb-run -a make check green: fmt, clippy with -D warnings, and every suite (export_pdf 135, tabs_smoke 165, plus the rest), 0 failures.
• New unit tests for the four presets (margins, line height, spacing, ornaments, title block), for the CSS parser (units, 1 to 4 value margins, colours, line height, family mapping, invalid input, comments, unknown properties reported once) and for the stylesheet overriding the template.
• An ignored integration test exports the same document without a stylesheet, with one, and without it again, rasterises page 1 with pdftoppm and asserts the first and third are identical and the second differs.
• The five presets were exported to PDF and reviewed as images: Custom and Article are identical, Manuscript gets 72 pt margins with a running title and page number, Letter 54 pt, Compact 36 pt with tighter spacing.