dev: URLs to files in public directories

Already existing was the functionality to go to puter.local/@username to
access the Public folder belonging to "username". This commit adds the
ability for relative paths such as puter.local/@username/document.md to
access /username/Public/document.md in the suggested app for handling
".md" files.

src/gui/src/UI/UIDesktop.js

@@ -1374,23 +1374,97 @@ async function UIDesktop(options) {
     //--------------------------------------------------------------------------------------
     const url_paths = window.location.pathname.split('/').filter(element => element);
     if (url_paths[0]?.startsWith('@')) {
-        let username = url_paths[0].substring(1);
+        const username = url_paths[0].substring(1);
         let item_path = '/' + username + '/Public';
-
-        // check if username has valid characters
-        if (!username.match(/^[a-z0-9_]+$/i)) {
-            UIAlert({
-                message: 'Invalid username.'
-            });
-        } else {
-            UIWindow({
-                path: item_path,
-                title: path.basename(item_path),
-                icon: await item_icon({ is_dir: true, path: item_path }),
-                is_dir: true,
-                app: 'explorer',
-            });
+        if ( url_paths.length > 1 ) {
+            item_path += '/' + url_paths.slice(1).join('/');
         }
+
+        // GUARD: avoid invalid user directories
+        {
+            if (!username.match(/^[a-z0-9_]+$/i)) {
+                UIAlert({
+                    message: 'Invalid username.'
+                });
+                return;
+            }
+        }
+
+        const stat = await puter.fs.stat(item_path);
+        console.log('stat result', stat);
+        
+        // TODO: DRY everything here with open_item. Unfortunately we can't
+        //       use open_item here because it's coupled with UI logic;
+        //       it requires a UIItem element and cannot operate on a
+        //       file path on its own.
+        if ( ! stat.is_dir ) {
+            if ( stat.associated_app ) {
+                launch_app({ name: stat.associated_app.name });
+                return;
+            }
+            
+            const ext_pref =
+                window.user_preferences[`default_apps${path.extname(item_path).toLowerCase()}`];
+            
+            if ( ext_pref ) {
+                launch_app({
+                    name: ext_pref,
+                    file_path: item_path,
+                });
+                return;
+            }
+            
+
+            const open_item_meta = await $.ajax({
+                url: window.api_origin + "/open_item",
+                type: 'POST',
+                contentType: "application/json",
+                data: JSON.stringify({
+                    path: item_path,
+                }),
+                headers: {
+                    "Authorization": "Bearer "+window.auth_token
+                },
+                statusCode: {
+                    401: function () {
+                        window.logout();
+                    },
+                },
+            });
+            const suggested_apps = open_item_meta?.suggested_apps ?? await window.suggest_apps_for_fsentry({
+                path: item_path
+            });
+
+            // Note: I'm not adding unzipping logic here. We'll wait until
+            //       we've refactored open_item so that Puter can have a
+            //       properly-reusable open function.
+            if ( suggested_apps.length !== 0 ) {
+                launch_app({
+                    name: suggested_apps[0].name, 
+                    token: open_item_meta.token,
+                    file_path: item_path,
+                    app_obj: suggested_apps[0],
+                    window_title: path.basename(item_path),
+                    maximized: options.maximized,
+                    file_signature: open_item_meta.signature,
+                });
+                return;
+            }
+
+            await UIAlert({
+                message: 'Cannot find an app to open this file; ' +
+                    'opening directory instead.'
+            });
+            item_path = item_path.split('/').slice(0, -1).join('/')
+        }
+
+        UIWindow({
+            path: item_path,
+            title: path.basename(item_path),
+            icon: await item_icon({ is_dir: true, path: item_path }),
+            is_dir: true,
+            app: 'explorer',
+        });
     }
 }