feat: configurable PDF export — page size, margins, orientation, page numbers

[sid8491]

Summary

Surfaces the print knobs the existing webContents.printToPDF call already supports (the pipeline was hardcoded to A4 + default margins) and adds two genuinely new things: landscape orientation and an optional "X / Y" page-number footer. All settings live under a new PDF export section in the existing Settings modal and persist to localStorage so each user only configures once.

Controls

Setting	Values
Page size	A4, Letter, Legal, A3, A5, Tabloid
Margins	Default (0.6in), Narrow (0.3in), None (0in)
Landscape orientation	toggle
Print background	toggle (off = force plain white)
Page numbers	toggle (centred footer, forces ≥0.5in vertical margin so footer doesn''t overlap content)

Wiring

• src/shared/types.ts — ExportPdfOptions gains landscape + displayPageNumbers; pageSize widens to the six Electron-supported sizes via a new PdfPageSize type.
• src/main/ipc/pdf.ts — threads new options into printToPDF. When displayPageNumbers is on we disable preferCSSPageSize (the @page rule otherwise wins over Chromium''s runtime footer area) and inject minimal header (<div></div>) + footer (centred Inter pageNumber / totalPages) templates.
• src/renderer/src/store/settings.ts — adds pdf: PdfPrefs with a single setPdf(partial) mutator; persisted alongside the existing theme/syncScroll/showLineNumbers fields.
• src/renderer/src/lib/exportPdf.ts — reads useSettings.getState().pdf and threads it into the IPC call. Ctrl+E and the command-palette entry continue to "just work".
• src/renderer/src/components/SettingsModal.tsx — new PdfExportSection rendered between About and AI provider, using the existing Field / Toggle / Select primitives so it matches the rest of the modal.

Tests

• New src/renderer/src/store/settings.test.ts covers defaults, setPdf partial-merge semantics, and localStorage round-trip.
• New vitest.setup.ts stubs window.matchMedia (jsdom doesn''t ship it) so any store that probes prefers-color-scheme at import time is safe to load in tests.
• Wired in via vitest.config.ts > test.setupFiles.

Test Files  6 passed (6)
     Tests  28 passed | 1 todo (29)

Test plan

• npm test, npm run lint, npm run typecheck, npm run format:check, npm run build all pass locally
• Manual UI verification still needed: open Settings, change each control, export PDF, confirm output respects the choice
• Especially: enable page numbers + narrow margins together and confirm the footer doesn''t collide with content (the auto-widen guard is for this)

Out of scope

• Per-export overrides (a modal before save with these same controls). Easy follow-up if there''s demand.
• Custom margin values (only presets right now).
• User-customisable header/footer templates beyond the simple page-number footer.
• Cover page / table of contents / outline embedding — bigger features in their own right.