feat(userlist): click a user to open chat with @<name> prefilled

Newcomers to a multi-user pad regularly fail to discover the chat
panel and the @-mention convention. Make the user list itself the
discovery affordance: clicking another user's row opens chat (if
hidden) and prefills the input with "@<their_name> ", ready to send.

The skin gets a small visual cue — pointer cursor on .usertdname and
an underline on hover — so the affordance is visible without
requiring a redesign. The color swatch keeps its own click semantics
(color picker), so the swatch cell is excluded from the new handler.

To let bot/AI plugins substitute their trigger string for an
otherwise-useless @-mention of the bot's display name (e.g.
"@AI Assistant" → "@ai"), this adds a new client-side hook,
chatPrefillFromUser, that takes {authorId, name, prefill} and lets
the first plugin to return a non-empty string override the default
prefill. Documented in doc/api/hooks_client-side.md alongside
chatSendMessage.

Plugin errors in the hook are caught — a misbehaving plugin can't
break the click. If chat is hidden by pad settings, chat.show() is
a no-op and the click effectively does nothing, which matches the
existing behavior of "no chat means no chat-related affordances".

The new prefill never clobbers a real partial message in the input;
if the user was mid-typing something, the @-mention is appended
rather than replacing.

doc/api/hooks_client-side.md

@@ -354,6 +354,34 @@ Context properties:
 
 * `message`: The message object that will be sent to the Etherpad server.
 
+## `chatPrefillFromUser`
+
+Called from: `src/static/js/pad_userlist.ts`
+
+Called when the user clicks an entry in the user list. The default behavior is to
+open the chat panel and prefill the input with `@<name> `, where `<name>` is that
+user's display name (with whitespace replaced by underscores). Plugins can return
+a different prefill string from their callback — the first non-empty string
+returned wins.
+
+Typical use is by AI/bot plugins whose author display name (e.g. "AI Assistant")
+isn't a useful @-mention; the plugin can substitute its trigger string instead.
+
+Context properties:
+
+* `authorId`: The clicked user's author id.
+* `name`: The clicked user's display name.
+* `prefill`: The default prefill string Etherpad would otherwise use.
+
+Example:
+
+```javascript
+exports.chatPrefillFromUser = (hookName, {authorId, name}, cb) => {
+  if (authorId === window.clientVars.ep_my_bot.authorId) return cb('@bot ');
+  return cb();
+};
+```
+
 ## collectContentPre
 
 Called from: `src/static/js/contentcollector.js`

src/static/js/pad_userlist.ts

@@ -19,6 +19,7 @@
 
 import padutils from './pad_utils'
 const hooks = require('./pluginfw/hooks');
+const chat = require('./chat').chat;
 import html10n from './vendors/html10n';
 let myUserInfo = {};
 
@@ -369,6 +370,46 @@ const paduserlist = (() => {
         }, 0);
       });
 
+      // Click any other user's row to open chat with @<their_name> prefilled.
+      // Helps newcomers discover the chat panel and the @-mention convention
+      // without having to be told. Plugins can transform the prefilled text
+      // — for example ep_ai_chat replaces "@AI Assistant" with the
+      // configured trigger ("@ai") — via the chatPrefillFromUser client
+      // hook (see doc/api/hooks_client-side.md).
+      $('#otheruserstable').on('click', 'tr[data-authorId]', async function (event) {
+        // Skip clicks on the color swatch — that has its own click handler
+        // (color-picker semantics) and shouldn't double up as a chat trigger.
+        if ($(event.target).closest('.usertdswatch').length) return;
+        const tr = $(this);
+        const authorId = tr.attr('data-authorId');
+        if (!authorId) return;
+        const name = (tr.find('.usertdname').text() || '').trim();
+        let prefill = name ? `@${name.replace(/\s+/g, '_')} ` : '';
+        try {
+          const transforms = await hooks.aCallAll(
+              'chatPrefillFromUser', {authorId, name, prefill});
+          if (Array.isArray(transforms)) {
+            for (const tr2 of transforms) {
+              if (typeof tr2 === 'string' && tr2.length > 0) { prefill = tr2; break; }
+            }
+          }
+        } catch { /* never let a misbehaving plugin break the click */ }
+        try { chat.show(); } catch { /* */ }
+        setTimeout(() => {
+          const $input = $('#chatinput');
+          if (!$input.length) return;
+          const current = ($input.val() || '') as string;
+          if (!current.trim() || /^@\S+\s*$/.test(current.trim())) {
+            $input.val(prefill);
+          } else if (!current.includes(prefill.trim())) {
+            $input.val(`${current.trimEnd()} ${prefill}`);
+          }
+          $input.trigger('focus');
+          const elem = $input[0] as HTMLTextAreaElement;
+          try { elem.setSelectionRange(elem.value.length, elem.value.length); } catch (_e) { /* */ }
+        }, 50);
+      });
+
       // color picker
       $('#myswatchbox').on('click', showColorPicker);
       $('#mycolorpicker .pickerswatchouter').on('click', function () {

src/static/skins/colibris/src/components/users.css

@@ -2,6 +2,19 @@ table#otheruserstable {
   margin-top: 20px;
 }
 
+/*
+ * Clicking a row in the user list opens chat and prefills "@<name> ".
+ * The pointer cursor on the name cell makes the affordance visible —
+ * the swatch keeps its own (color-picker) click semantics, so leave
+ * its cursor alone.
+ */
+#otheruserstable tr[data-authorId] .usertdname {
+  cursor: pointer;
+}
+#otheruserstable tr[data-authorId]:hover .usertdname {
+  text-decoration: underline;
+}
+
 .popup#users.chatAndUsers > .popup-content {
   padding: 20px 10px;
   height: 250px;