feat(userlist): click a user to open chat with @<name> prefilled (#7660)

* 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.

* fix(userlist): don't steal rename focus + add Playwright coverage

Two follow-ups on review of the click-to-chat handler:

1. Bug (Qodo, correctness): clicking the rename <input> on an unnamed
   user's row triggered the new row handler, which then focused
   #chatinput and made it impossible to name unnamed users from the
   user list. Add an early-return that skips form controls inside
   the row (input/textarea/select/button/a/[contenteditable=true]).
   The swatch was already excluded; this widens the same idea to
   anything that's interactive on its own merits.

2. Test coverage: add a frontend Playwright spec
   (userlist_click_to_chat.spec.ts) covering the supported flows
   and the new regression:
   - clicking another named user opens chat and prefills "@<name> "
   - clicking the swatch opens color picker, not chat
   - clicking the rename <input> on an unnamed user keeps focus
     on the input (regression test for the bug above)
   - partial chat message is preserved when prefilling

* test: stabilise the partial-message preservation case

The 'partial message in chat input is preserved when prefilling'
case was flaking on CI. Three small changes:

- Seed the chat input with fill() rather than click() + keyboard.type().
  Earlier the test was racing chat.focus()'s own setTimeout(100) — when
  the keyboard.type started before that timer fired, the typing landed
  in whatever element had focus at the time, which wasn't always the
  chat input. fill() bypasses focus state entirely.
- Wait for the chat box to be visible before filling, so we don't race
  the chaticon click handler.
- Replace the two sequential expect/wait pairs after the daveRow click
  with one waitForFunction that asserts both 'hi there' and '@Dave' are
  in the input together. The prefill is async (setTimeout(50) inside
  the click handler), so a combined wait is more reliable than checking
  one piece, then snapshotting and asserting the other.

The other three cases in this file passed unchanged on CI; only this
fourth one was racy.

* fix: don't commit local .claude worktrees / var state

These were accidentally added in ffe947706 by an over-broad git add -A.
Both paths are workspace-local and unrelated to this PR.

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`