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/main.py

@@ -39,6 +39,8 @@ async def lifespan(app: FastAPI):
         await _add_column_if_missing(conn, "children", "strikes", "INT NOT NULL DEFAULT 0")
         await _add_column_if_missing(conn, "users", "timezone", "VARCHAR(64) NOT NULL DEFAULT 'UTC'")
         await _add_column_if_missing(conn, "subjects", "is_system", "TINYINT(1) NOT NULL DEFAULT 0")
+        await _add_column_if_missing(conn, "users", "last_active_at", "DATETIME NULL")
+        await _add_column_if_missing(conn, "children", "strikes_last_reset", "DATE NULL")
 
     # Seed Meeting system subject for any existing users who don't have one
     from app.database import AsyncSessionLocal