[Designer] #342 UX round 2 — chat 빈 상태 + 한국식 날짜 + 표현 방식

남은 P1/P2 3건.

- ChatScreen 빈 상태: 아이콘 + 한 줄 설명 + 예시 prompt 4개
  (tap → _textCtrl 자동 채움, 자동 send X).
- CheckIn 날짜: '2026-06-15' raw → '6월 15일 (월)' 한국식.
  DB 저장은 _ymd 유지.
- HabitCreate '프레임 레벨' → '표현 방식' + helperText.
  아이템: '조건부 행동 (예: 아침에 햇빛 받기)' / '정체성 (예: 나는 일찍 자는 사람)'.
- 설계서 #342 README — D 섹션 + AC-D1/D2/D3 추가.
- CHANGELOG v0.4.2 UX round 2 블록.

167 tests passed, analyze clean.

Refs #342

app/lib/ui/screens/chat_screen.dart

@@ -132,24 +132,31 @@ class _ChatScreenState extends ConsumerState<ChatScreen> {
             ),
           ),
         Expanded(
-          child: ListView.builder(
-            controller: _scrollCtrl,
-            padding: const EdgeInsets.all(12),
-            itemCount: state.messages.length +
-                (state.streamingText != null &&
-                        state.streamingText!.isNotEmpty
-                    ? 1
-                    : 0),
-            itemBuilder: (context, i) {
-              if (i < state.messages.length) {
-                return _MessageBubble(message: state.messages[i]);
-              }
-              return _MessageBubble(
-                message: ModelChatMessage(state.streamingText ?? ''),
-                streaming: true,
-              );
-            },
-          ),
+          child: state.messages.isEmpty && state.streamingText == null
+              ? _EmptyChatHint(onPickPrompt: (p) {
+                  _textCtrl.text = p;
+                  _textCtrl.selection = TextSelection.fromPosition(
+                    TextPosition(offset: p.length),
+                  );
+                })
+              : ListView.builder(
+                  controller: _scrollCtrl,
+                  padding: const EdgeInsets.all(12),
+                  itemCount: state.messages.length +
+                      (state.streamingText != null &&
+                              state.streamingText!.isNotEmpty
+                          ? 1
+                          : 0),
+                  itemBuilder: (context, i) {
+                    if (i < state.messages.length) {
+                      return _MessageBubble(message: state.messages[i]);
+                    }
+                    return _MessageBubble(
+                      message: ModelChatMessage(state.streamingText ?? ''),
+                      streaming: true,
+                    );
+                  },
+                ),
         ),
         const Divider(height: 1),
         Padding(
@@ -248,6 +255,73 @@ class _WarmupErrorBanner extends ConsumerWidget {
   }
 }
 
+/// 첫 진입 시 빈 대화창 안내. 사용자가 무엇을 물어볼 수 있는지
+/// 예시 prompt 4개 + tool 카테고리 짧은 안내. tap 하면 입력창에 채워지고
+/// 사용자가 보내기만 누르면 됨 (자동 send X — 사용자가 문구 수정 여지).
+class _EmptyChatHint extends StatelessWidget {
+  final ValueChanged<String> onPickPrompt;
+  const _EmptyChatHint({required this.onPickPrompt});
+
+  static const _examples = [
+    '아침 햇빛 받기 습관 추가해줘',
+    '오늘 운동 했어',
+    '내 스트릭 보여줘',
+    '수면 프로토콜 알려줘',
+  ];
+
+  @override
+  Widget build(BuildContext context) {
+    final theme = Theme.of(context);
+    return SingleChildScrollView(
+      padding: const EdgeInsets.symmetric(horizontal: 24, vertical: 32),
+      child: Column(
+        crossAxisAlignment: CrossAxisAlignment.stretch,
+        children: [
+          Icon(
+            Icons.smart_toy_outlined,
+            size: 48,
+            color: theme.colorScheme.primary,
+          ),
+          const SizedBox(height: 16),
+          Text(
+            'AI 코치',
+            style: theme.textTheme.titleLarge,
+            textAlign: TextAlign.center,
+          ),
+          const SizedBox(height: 8),
+          Text(
+            '습관 추가·기록 · 카탈로그 검색 · 스트릭 조회를 도와드려요.',
+            style: theme.textTheme.bodyMedium?.copyWith(
+              color: theme.colorScheme.onSurfaceVariant,
+            ),
+            textAlign: TextAlign.center,
+          ),
+          const SizedBox(height: 24),
+          Text(
+            '예시',
+            style: theme.textTheme.labelLarge?.copyWith(
+              color: theme.colorScheme.onSurfaceVariant,
+            ),
+          ),
+          const SizedBox(height: 8),
+          ..._examples.map((p) => Padding(
+                padding: const EdgeInsets.symmetric(vertical: 4),
+                child: OutlinedButton(
+                  onPressed: () => onPickPrompt(p),
+                  style: OutlinedButton.styleFrom(
+                    alignment: Alignment.centerLeft,
+                    padding: const EdgeInsets.symmetric(
+                        horizontal: 16, vertical: 12),
+                  ),
+                  child: Text(p),
+                ),
+              )),
+        ],
+      ),
+    );
+  }
+}
+
 /// Human-friendly Korean labels for the 6 tools registered in
 /// `ToolRegistry.defaults()`. Falls back to the raw tool name for any
 /// future tool that hasn't been mapped yet — better to show the raw id