Add single-instance support and clean up scaffolding files

Ensure only one Vesper instance runs at a time. When a second
instance is launched with a file argument, the file is forwarded
to the existing window. Remove unused template SVGs and test file.

src-tauri/Cargo.toml

@@ -22,3 +22,6 @@ tauri-plugin-fs = "2"
 serde = { version = "1", features = ["derive"] }
 serde_json = "1"
 
+[target.'cfg(not(any(target_os = "android", target_os = "ios")))'.dependencies]
+tauri-plugin-single-instance = "2"
+