Read picked import files through the fs plugin so Android content URIs work

On Android the open dialog returns a content:// URI, which std::fs can't
open, so ManaBox/library and deck imports failed with "no such file or
directory". Read them via tauri-plugin-fs, which resolves content URIs and
falls through to plain paths on desktop.

Closes #24

src-tauri/Cargo.toml

@@ -16,6 +16,9 @@ tauri-build = { version = "2", features = [] }
 [dependencies]
 tauri = { version = "2", features = [] }
 tauri-plugin-dialog = "2"
+# Reads the files the dialog picks: on Android those are content:// URIs, which
+# std::fs can't open.
+tauri-plugin-fs = "2"
 serde = { version = "1", features = ["derive"] }
 serde_json = "1"
 mtgc-core = { path = "../crates/mtgc-core" }