Add Done button, tablet controls, super admin management, midnight strike reset, and activity log improvements

- Done button snaps block to full duration, marks complete, logs "Marked Done by User"; Reset after Done fully un-completes the block
- Session action buttons stretch full-width and double height for tablet tapping
- Super admin: reset password, disable/enable accounts, delete user (with cascade), last active date per user's timezone
- Disabled account login returns specific error message instead of generic invalid credentials
- Users can change own password from Admin → Settings
- Strikes reset automatically at midnight in user's configured timezone (lazy reset on page load)
- Break timer state fully restored when navigating away and back to dashboard
- Timer no longer auto-starts on navigation if it wasn't running before
- Implicit pause guard: no duplicate pause events when switching already-paused blocks or starting a break
- Block selection events removed from activity log; all event types have human-readable labels
- House emoji favicon via inline SVG data URI
- README updated to reflect all changes

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/models/child.py

@@ -1,6 +1,7 @@
 from datetime import date
 from sqlalchemy import String, Boolean, ForeignKey, Date, Integer
 from sqlalchemy.orm import Mapped, mapped_column, relationship
+from typing import Optional
 from app.models.base import Base, TimestampMixin
 
 
@@ -14,6 +15,7 @@ class Child(TimestampMixin, Base):
     is_active: Mapped[bool] = mapped_column(Boolean, default=True)
     color: Mapped[str] = mapped_column(String(7), default="#4F46E5")  # hex color for UI
     strikes: Mapped[int] = mapped_column(Integer, default=0, nullable=False)
+    strikes_last_reset: Mapped[Optional[date]] = mapped_column(Date, nullable=True, default=None)
 
     user: Mapped["User"] = relationship("User", back_populates="children")  # noqa: F821
     daily_sessions: Mapped[list["DailySession"]] = relationship(  # noqa: F821

backend/app/models/user.py

@@ -1,4 +1,5 @@
-from sqlalchemy import String, Boolean
+from datetime import datetime
+from sqlalchemy import String, Boolean, DateTime
 from sqlalchemy.orm import Mapped, mapped_column, relationship
 from app.models.base import Base, TimestampMixin
 
@@ -13,6 +14,7 @@ class User(TimestampMixin, Base):
     is_active: Mapped[bool] = mapped_column(Boolean, default=True)
     is_admin: Mapped[bool] = mapped_column(Boolean, default=False)
     timezone: Mapped[str] = mapped_column(String(64), nullable=False, default="UTC")
+    last_active_at: Mapped[datetime | None] = mapped_column(DateTime, nullable=True, default=None)
 
     children: Mapped[list["Child"]] = relationship("Child", back_populates="user")  # noqa: F821
     subjects: Mapped[list["Subject"]] = relationship("Subject", back_populates="user")  # noqa: F821