Add Rust logo pipeline: trim, metrics, knockout and Branding settings

Logos (PNG, JPEG, WebP) are decoded, trimmed to their ink box, downscaled
to 1200 px and re-encoded as a content-addressed print PNG plus a white
knockout variant when the logo has alpha; aspect, ink density, mean colour
and kind (wordmark, mark, tall) are stored with the settings and frozen in
the issue snapshot. Legacy logos are derived at startup. The Arun P logo
PDF shrinks from 334 KB to 92 KB. Settings gains a Branding section with
white and dark previews and a business-name toggle (migration M5).

src/lib/previewInvoice.ts

@@ -40,6 +40,9 @@ export function vendorSnapshotJson(settings: Settings): string {
     vendorGstin: settings.gstRegistration === "unregistered" ? "" : settings.vendorGstin.trim(),
     vendorStateCode: settings.vendorStateCode,
     logoPath: settings.logoPath,
+    logoKnockoutPath: settings.logoKnockoutPath,
+    logoMeta: settings.logoMeta,
+    logoIncludesName: settings.logoIncludesName,
     signaturePath: settings.signaturePath,
     gstRegistration: settings.gstRegistration,
     signatoryName: settings.signatoryName,