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-tauri/Cargo.toml

@@ -28,6 +28,7 @@ thiserror = "2"
 base64 = "0.22"
 sha2 = "0.10"
 percent-encoding = "2"
+image = { version = "0.25.10", default-features = false, features = ["png", "jpeg", "webp"] }
 
 # Desktop-only plugins (not available on mobile targets).
 [target.'cfg(not(any(target_os = "android", target_os = "ios")))'.dependencies]