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 Test Vendor 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/src/commands/assets.rs

@@ -78,6 +78,24 @@ fn write_asset(data_dir: &Path, kind: &str, bytes: &[u8]) -> Result<String, Stri
     Ok(format!("assets/{file}"))
 }
 
+/// Write `assets/<file_name>` unless it already exists. For content-addressed derivatives, where the
+/// same name always means the same bytes. The name must be a plain `[a-z0-9._-]` file name.
+pub fn write_named_asset(data_dir: &Path, file_name: &str, bytes: &[u8]) -> Result<String, String> {
+    let plain = !file_name.is_empty()
+        && !file_name.starts_with('.')
+        && file_name
+            .chars()
+            .all(|c| c.is_ascii_lowercase() || c.is_ascii_digit() || matches!(c, '.' | '-' | '_'));
+    if !plain {
+        return Err(format!("Invalid asset name: {file_name}"));
+    }
+    let target = assets_dir(data_dir)?.join(file_name);
+    if !target.exists() {
+        std::fs::write(&target, bytes).map_err(|e| e.to_string())?;
+    }
+    Ok(format!("assets/{file_name}"))
+}
+
 fn asset_data_uri(data_dir: &Path, path: &str) -> Result<String, String> {
     let file = resolve_asset(data_dir, path)?;
     let bytes = std::fs::read(&file).map_err(|e| e.to_string())?;
@@ -162,6 +180,18 @@ mod tests {
         assert!(uri.starts_with("data:image/jpeg;base64,"));
     }
 
+    #[test]
+    fn named_assets_are_plain_names_and_not_overwritten() {
+        let dir = tempdir().unwrap();
+        let stored = write_named_asset(dir.path(), "logo-ab12cd34-print.png", b"one").unwrap();
+        assert_eq!(stored, "assets/logo-ab12cd34-print.png");
+        write_named_asset(dir.path(), "logo-ab12cd34-print.png", b"two").unwrap();
+        assert_eq!(std::fs::read(dir.path().join(&stored)).unwrap(), b"one");
+        for bad in ["", ".hidden", "../x.png", "a/b.png", "A.png", "x y.png"] {
+            assert!(write_named_asset(dir.path(), bad, b"x").is_err(), "{bad}");
+        }
+    }
+
     #[test]
     fn data_uri_mime_comes_from_the_bytes_not_the_extension() {
         let dir = tempdir().unwrap();