doc: added documentation for CORS error mitigation during ui development

Previously, the playlist_filebrowser always contained a 'cancel' and
'add tracks' button, because it was used in context of the playlist
editor only.

The playlist_filebrowser now differentiates between the intents it is
being used with. In context of the new filesystem browser button, the
'cancel' and 'add tracks' buttons are actively hidden.

DEVELOP.md

@@ -52,3 +52,23 @@ would be stored in the following key/value pairs in the btree db:
  * 00aa11bb22/playlist/00000: a.mp3
  * 00aa11bb22/playlist/00001: b.mp3
  * 00aa11bb22/playlistpos: 00000
+
+## Notes for UI development with chromium
+
+Features for the web interface are best prototyped in a browser directly. By using the built-in developmer tools and
+and their "override" feature, the web contents are replaced by a locally stored copy, which can be used to directly
+test the modifications without going all the way through the build and flash process.
+
+However, modern browsers may restrict or even completely forbid the execution of dynamic content like JavaScript, if
+the content is stored on the local machine and/or the content is accessed using http. In such a case, chromium issues
+an error message similar to the following one:
+
+> Access to fetch at 'http://192.168.4.1/api/v1/audiofiles' from origin 'http://192.168.4.1' has been blocked by CORS
+> policy: The request client is not a secure context and the resource is in more-private address space `local`.
+
+To mitigate this, chromium offers two flags that need modification:
+- 'chrome://flags/#local-network-access-check' must be `Disabled`
+- 'chrome://flags/#unsafely-treat-insecure-origin-as-secure' must be `Enabled`
+
+Note that these settings leave the browser susceptible to security issues and should be returned to
+their default values as soon as possible.

software/frontend/index.html

@@ -147,6 +147,7 @@
     <button onclick="showScreen('menu')">🏠 Main Menu</button>
     <button onclick="showScreen('config')">⚙️ Config Editor</button>
     <button onclick="showScreen('playlist')">🖹 Playlist Editor</button>
+    <button onclick="showScreen('playlist_filebrowser', 'filesystem')">📂 Filesystem</button>
 </nav>
 
 <!-- MAIN MENU -->
@@ -243,7 +244,7 @@
 
 <!-- PLAYLIST EDITOR SCREEN 3: file browser -->
 <div id="screen-playlist_filebrowser" class="screen">
-    <h2>Playlist Editor</h2>
+    <h2 id="playlist-filebrowser-title">Playlist Editor</h2>
     <div id="playlist-filebrowser-container">
         <div class="scroll-container">
             <div class="tree" id="playlist-filebrowser-tree">
@@ -787,6 +788,8 @@
        PLAYLIST EDITOR LOGIC - ADD FILES SCREEN
     ------------------------------------------- */
     Screens.playlist_filebrowser = (() => {
+        let isFilesystemMode = false;
+
         function init() {
             document.getElementById('playlist-filebrowser-cancel').addEventListener("click", (e) => {
                 showScreen("playlist_edit", {});
@@ -805,13 +808,32 @@
             });
             tree.init();
         }
-        
+
         async function onShow(intent) {
+            console.log(intent)
             document.getElementById('playlist-filebrowser-addtrack').disabled = true;
+
+            const title = document.getElementById('playlist-filebrowser-title');
+            const cancelButton = document.getElementById('playlist-filebrowser-cancel');
+            const addTracksButton = document.getElementById('playlist-filebrowser-addtrack');
+
             if (intent !== 'refresh') {
+                isFilesystemMode = (intent === 'filesystem');
+
                 document.getElementById('playlist-filebrowser-upload-progress').value = 0;
                 document.getElementById("playlist-filebrowser-upload-files").value = "";
             }
+
+            if (isFilesystemMode) {
+                title.innerText = "Filesystem";
+                cancelButton.style.display = 'none';
+                addTracksButton.style.display = 'none';
+            } else {
+                title.innerText = "Playlist Editor";
+                cancelButton.style.display = ''
+                addTracksButton.style.display = ''
+            }
+
             tree = document.getElementById("playlist-filebrowser-tree");
             tree.innerHTML = "Loading...";
             fetch('/api/v1/audiofiles')