feat: replace inline online dialog with reusable dialog components and add help modal

Extract the online users dialog from `online.html` into a dedicated `dialog_online.html` template, create a new `dialog_help.html` template with a custom `help-command-list` web component, and add global `<dialog>` styling in `base.css` with centered positioning, backdrop blur, and fade/scale animations. Update `web.html` to include both dialog templates, wire the `/help` autocomplete command to `showHelp()`, and rename the `/online` handler from `showOnlineUsers()` to `showOnline()`.

src/snek/templates/web.html

@@ -21,7 +21,8 @@
         <upload-button channel="{{ channel.uid.value }}"></upload-button>
     </div>
 </section>
-{% include "online.html" %}
+{% include "dialog_help.html" %}
+{% include "dialog_online.html" %}
 <script type="module">
     import { app } from "/app.js";
     import { Schedule } from "/schedule.js";
@@ -32,13 +33,16 @@
     }
     getInputField().autoComplete = {
         "/online": () =>{
-            showOnlineUsers();
+            showOnline();
         },
         "/clear": () => {
             document.querySelector(".chat-messages").innerHTML = '';
         },
         "/live": () =>{
             getInputField().liveType = !getInputField().liveType
+        },
+        "/help": () => {
+            showHelp();
         }
     }