FEAT: improve conversation history loading for long sessions (#371)

* feat: add "Load all messages" button for long conversations

Scrolling up through long conversations requires many "load more" cycles.
This adds a "Load all messages" floating button that fetches the entire
conversation history in one shot.

- Floating overlay pill appears after each batch finishes loading, persists 2s
- Shows loading spinner while fetching all messages
- Shows green "All messages loaded" confirmation for 1s before disappearing
- Preserves scroll position when bulk-loading (no viewport jump)
- Ref-based guards prevent scroll handler from re-fetching after load-all
- Performance warning shown; "Scroll to bottom" resets visible cap
- Clean state reset on session switch
- i18n keys for en and zh-CN

Note: default page size (20) and visible cap (100) are unchanged.
These could be increased in a follow-up or made configurable via settings.

* re-implement load-all feature for new TS architecture

src/i18n/locales/en/chat.json

@@ -175,7 +175,11 @@
       "showingOf": "Showing {{shown}} of {{total}} messages",
       "scrollToLoad": "Scroll up to load more",
       "showingLast": "Showing last {{count}} messages ({{total}} total)",
-      "loadEarlier": "Load earlier messages"
+      "loadEarlier": "Load earlier messages",
+      "loadAll": "Load all messages",
+      "loadingAll": "Loading all messages...",
+      "allLoaded": "All messages loaded",
+      "perfWarning": "All messages loaded — scrolling may be slower. Click \"Scroll to bottom\" to restore performance."
     }
   },
   "shell": {

src/i18n/locales/ko/chat.json

@@ -175,7 +175,11 @@
       "showingOf": "{{total}}개 중 {{shown}}개 표시",
       "scrollToLoad": "위로 스크롤하여 더 로드",
       "showingLast": "마지막 {{count}}개 메시지 표시 (총 {{total}}개)",
-      "loadEarlier": "이전 메시지 로드"
+      "loadEarlier": "이전 메시지 로드",
+      "loadAll": "모든 메시지 로드",
+      "loadingAll": "모든 메시지 로딩 중...",
+      "allLoaded": "모든 메시지 로드 완료",
+      "perfWarning": "모든 메시지가 로드됨 - 스크롤이 느려질 수 있습니다. \"맨 아래로 스크롤\"을 클릭하면 성능이 복구됩니다."
     }
   },
   "shell": {

src/i18n/locales/zh-CN/chat.json

@@ -175,7 +175,11 @@
       "showingOf": "显示 {{shown}} / {{total}} 条消息",
       "scrollToLoad": "向上滚动以加载更多",
       "showingLast": "显示最近 {{count}} 条消息（共 {{total}} 条）",
-      "loadEarlier": "加载更早的消息"
+      "loadEarlier": "加载更早的消息",
+      "loadAll": "加载全部消息",
+      "loadingAll": "正在加载全部消息...",
+      "allLoaded": "全部消息已加载",
+      "perfWarning": "已加载全部消息 - 滚动可能变慢。点击「滚动到底部」恢复性能。"
     }
   },
   "shell": {