chat: stick-to-bottom auto-scroll with jump-to-latest button

Auto-scroll now yields when the reader scrolls away from the bottom, so a
fast-streaming reply no longer fights someone reading the start. Stickiness is
a flag driven by a passive scroll listener (not a per-flush distance check,
which breaks when one flush adds more than the threshold of content); scrolling
back within the band re-arms it.

Centralised in chat-session.js (_scrollToBottom(force) + _forceScrollToBottom
+ _messagesContainer hook), removing the duplicated overrides in copilot.js /
chat-page.js. Force-scroll is used where the latest must be shown: history
load, chat reopen, approval/clarification prompts, and message send.

A sticky 'jump to latest' affordance appears only while scrolled up.

web/i18n/en.js

@@ -68,6 +68,7 @@ export default {
   'chat.reasoning':     'Reasoning…',
   'chat.attach':        'Attach files',
   'chat.new_session':   'New conversation',
+  'chat.scroll_to_latest': 'Scroll to latest',
   'chat.security_group': 'Security group',
   'chat.collapse':      'Hide chat',
   'chat.close_tab':     'Close tab',

web/i18n/fr.js

@@ -68,6 +68,7 @@ export default {
   'chat.reasoning':     'Raisonnement…',
   'chat.attach':        'Joindre des fichiers',
   'chat.new_session':   'Nouvelle conversation',
+  'chat.scroll_to_latest': 'Aller aux derniers messages',
   'chat.security_group': 'Groupe de sécurité',
   'chat.collapse':      'Masquer la discussion',
   'chat.close_tab':     'Fermer l\'onglet',

web/i18n/it.js

@@ -68,6 +68,7 @@ export default {
   'chat.reasoning':     'Ragionamento…',
   'chat.attach':        'Allega file',
   'chat.new_session':   'Nuova conversazione',
+  'chat.scroll_to_latest': 'Vai agli ultimi messaggi',
   'chat.security_group': 'Gruppo di sicurezza',
   'chat.collapse':      'Nascondi la chat',
   'chat.close_tab':     'Chiudi scheda',