refactor: externalize local upload helpers

Add local-upload-runtime.js as a browser runtime module for upload and attachment helper functions, with sidebar fallback wrappers and a fixed runtime asset route.

While validating task479, fix the editor attachment delete path so DOM-selected local attachment links are mapped back to a Tiptap text selection before delete_selection(), preserving undo history and adjacent links.

rust/crates/mnote-web/src/routes/mod.rs

@@ -98,6 +98,10 @@ pub fn build_router(state: AppState) -> Router {
             "/api/mnote-browser-runtime/resource-open-runtime.js",
             get(web_shell::resource_open_runtime_asset),
         )
+        .route(
+            "/api/mnote-browser-runtime/local-upload-runtime.js",
+            get(web_shell::local_upload_runtime_asset),
+        )
         .route(
             "/api/mnote-browser-runtime/tree-live-controller.js",
             get(web_shell::tree_live_controller_runtime_asset),

rust/crates/mnote-web/src/routes/web_shell.rs

@@ -4381,6 +4381,20 @@ pub async fn resource_open_runtime_asset() -> Response {
         .unwrap_or_else(|_| Response::new(Body::empty()))
 }
 
+pub async fn local_upload_runtime_asset() -> Response {
+    const JS: &str = include_str!("../../browser/local-upload-runtime.js");
+    Response::builder()
+        .status(StatusCode::OK)
+        .header(
+            header::CONTENT_TYPE,
+            "application/javascript; charset=utf-8",
+        )
+        .header(header::CACHE_CONTROL, "no-store")
+        .header(HEADER_MNOTE_WEB_OWNER, "mnote-web")
+        .body(Body::from(JS))
+        .unwrap_or_else(|_| Response::new(Body::empty()))
+}
+
 pub async fn tree_live_controller_runtime_asset() -> Response {
     const JS: &str = include_str!("../../browser/tree-live-controller.js");
     Response::builder()