add habit leveling/XP system, explicit fail marking for missed habits, and scrollable habit tabs

- Habits and the account now accumulate XP and levels from check streaks, with
  streak/level-up celebration banners and push notifications.
- The "yesterday missed" banner on /today now lets a habit be explicitly marked
  failed (not just checked done), backed by a new HabitLog.status column so
  completion-rate/streak calculations never count a failed day as done.
- The habit management tabs scroll horizontally on narrow screens instead of
  wrapping to two lines, with a pure-CSS edge shadow indicating more content.

migrations/versions/0009_add_habit_log_status.py

@@ -0,0 +1,32 @@
+"""add status (done/failed) to habit_log
+
+Revision ID: 0009_add_habit_log_status
+Revises: 0008_normalize_habit_enum_casing
+Create Date: 2026-07-20
+
+"어제 놓친 습관" 배너에서 사용자가 명시적으로 실패를 확정할 수 있게 하기 위해, "로그 행이 존재하면
+그 날 체크 완료"라는 기존 불변식을 유지하면서도 "완료"와 "실패"를 구분해야 한다. status 컬럼을
+추가하고 기존 행은 전부 완료였으므로 기본값 'done'으로 채운다.
+"""
+from typing import Sequence, Union
+
+import sqlalchemy as sa
+from alembic import op
+
+revision: str = "0009_add_habit_log_status"
+down_revision: Union[str, None] = "0008_normalize_habit_enum_casing"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.add_column(
+        "habit_log",
+        sa.Column("status", sa.String(length=20), nullable=False, server_default="done"),
+    )
+    op.create_check_constraint("ck_habit_log_status", "habit_log", "status in ('done','failed')")
+
+
+def downgrade() -> None:
+    op.drop_constraint("ck_habit_log_status", "habit_log", type_="check")
+    op.drop_column("habit_log", "status")