fix: Add CORS proxy fallback for Nextcloud WebDAV connections

- Add NEXTCLOUD_PROXY constant for corsproxy.io
- Add getProxiedUrl() method to NextcloudStorage class
- Add fetchWithProxy() wrapper method that tries direct connection first,
  then falls back to CORS proxy if direct connection fails
- Update all fetch() calls in NextcloudStorage to use fetchWithProxy()
- Add useProxy flag to track proxy usage
- This fixes 'NetworkError when attempting to fetch resource' for servers
  like wralto.org/nextcloud3 that have CORS restrictions

Co-authored-by: trevor1969 <trevor1969@users.noreply.github.com>

app.js

@@ -12,6 +12,9 @@ const STORAGE_KEY_NC_CONFIG = 'teatracker_nextcloud_config';
 // Nextcloud Storage Integration
 // ============================================
 
+// CORS Proxy for Nextcloud WebDAV (fallback when direct connection fails due to CORS)
+const NEXTCLOUD_PROXY = 'https://corsproxy.io/?';
+
 class NextcloudStorage {
     constructor(baseUrl, username, password, path = '/TeeTracker/') {
         this.baseUrl = baseUrl.replace(/\/$/, '');
@@ -21,6 +24,7 @@ class NextcloudStorage {
         this.authHeader = 'Basic ' + btoa(`${username}:${password}`);
         this.connected = false;
         this.lastError = null;
+        this.useProxy = false;
     }
 
     getFileUrl(filename) {
@@ -30,10 +34,54 @@ class NextcloudStorage {
     getDirectoryUrl() {
         return `${this.baseUrl}/remote.php/dav/files/${encodeURIComponent(this.username)}${this.path}`;
     }
+    // Get proxied URL for CORS fallback
+    getProxiedUrl(url) {
+        return `${NEXTCLOUD_PROXY}${encodeURIComponent(url)}`;
+    }
+
+    // Wrapper for fetch that automatically uses proxy on CORS failure
+    async fetchWithProxy(url, options = {}) {
+        try {
+            // First try direct connection
+            const response = await fetch(url, options);
+            
+            // If we get a CORS error or network error, it will throw
+            // If we get here, the request succeeded (even if status is not 2xx)
+            if (response.ok || response.status === 404 || response.status === 405) {
+                this.useProxy = false;
+                return response;
+            }
+            
+            // Some errors might still occur, so try proxy
+            console.log('Direct connection returned non-ok status, trying proxy...');
+        } catch (error) {
+            // Direct connection failed (CORS or network error), try proxy
+            console.log('Direct connection failed, trying proxy...');
+        }
+        
+        // Try with proxy
+        try {
+            const proxyUrl = this.getProxiedUrl(url);
+            this.useProxy = true;
+            const response = await fetch(proxyUrl, options);
+            
+            if (response.ok || response.status === 404 || response.status === 405) {
+                console.log('Proxy connection successful');
+                return response;
+            }
+            
+            // If proxy also fails, throw error
+            throw new Error(`Proxy returned status: ${response.status}`);
+        } catch (proxyError) {
+            this.useProxy = false;
+            console.error('Proxy connection also failed:', proxyError);
+            throw new Error(`Both direct and proxy connections failed: ${error.message || ''} ${proxyError.message || ''}`);
+        }
+    }
 
     async testConnection() {
         try {
-            const response = await fetch(this.getDirectoryUrl(), {
+            const response = await this.fetchWithProxy(this.getDirectoryUrl(), {
                 method: 'PROPFIND',
                 headers: {
                     'Authorization': this.authHeader,
@@ -65,7 +113,7 @@ class NextcloudStorage {
 
     async ensureDirectory() {
         try {
-            const response = await fetch(this.getDirectoryUrl(), {
+            const response = await this.fetchWithProxy(this.getDirectoryUrl(), {
                 method: 'MKCOL',
                 headers: { 'Authorization': this.authHeader }
             });
@@ -80,7 +128,7 @@ class NextcloudStorage {
     async ensureSubDirectory(subPath) {
         try {
             const subDirUrl = `${this.getDirectoryUrl()}${subPath}/`;
-            const response = await fetch(subDirUrl, {
+            const response = await this.fetchWithProxy(subDirUrl, {
                 method: 'MKCOL',
                 headers: { 'Authorization': this.authHeader }
             });
@@ -94,7 +142,7 @@ class NextcloudStorage {
     async loadFile(filename) {
         try {
             const url = this.getFileUrl(filename);
-            const response = await fetch(url, {
+            const response = await this.fetchWithProxy(url, {
                 headers: { 'Authorization': this.authHeader }
             });
 
@@ -114,7 +162,7 @@ class NextcloudStorage {
     async saveFile(filename, data) {
         try {
             const url = this.getFileUrl(filename);
-            const response = await fetch(url, {
+            const response = await this.fetchWithProxy(url, {
                 method: 'PUT',
                 headers: {
                     'Authorization': this.authHeader,
@@ -132,7 +180,7 @@ class NextcloudStorage {
     async fileExists(filename) {
         try {
             const url = this.getFileUrl(filename);
-            const response = await fetch(url, {
+            const response = await this.fetchWithProxy(url, {
                 method: 'HEAD',
                 headers: { 'Authorization': this.authHeader }
             });