Add project Schedule page with interactive Gantt chart

Adds a Schedule tab (Kitsu-style production schedule) under each
project: tasks grouped by type with drag-to-reschedule bars, Day/Week/
Month zoom, manual date-range control, weekend shading, a frozen task
column, and a two-tier month/date axis header. Requires a new
start_date field on Task (start_date was previously missing; only
deadline existed) and shadcn DatePicker inputs replace native date
inputs on the Schedule toolbar and TaskDetailPanel's Start Date/
Deadline fields.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/migrate_task_start_date.py

@@ -0,0 +1,85 @@
+#!/usr/bin/env python3
+"""
+Migration script to add the start_date column to the tasks table.
+
+Usage:
+    python migrate_task_start_date.py
+"""
+
+import sqlite3
+import sys
+from pathlib import Path
+
+
+def get_database_path():
+    """Get the database path, trying multiple possible locations."""
+    possible_paths = [
+        "vfx_project_management.db",  # Primary database
+        "database.db",
+        "../vfx_project_management.db"
+    ]
+
+    for path in possible_paths:
+        if Path(path).exists():
+            return path
+
+    return "vfx_project_management.db"
+
+
+def check_column_exists(cursor, table_name, column_name):
+    """Check if a column exists in a table."""
+    cursor.execute(f"PRAGMA table_info({table_name})")
+    columns = [column[1] for column in cursor.fetchall()]
+    return column_name in columns
+
+
+def migrate_database():
+    """Add start_date column to the tasks table."""
+    db_path = get_database_path()
+    print(f"Using database: {db_path}")
+
+    conn = None
+    try:
+        conn = sqlite3.connect(db_path)
+        cursor = conn.cursor()
+
+        cursor.execute("SELECT name FROM sqlite_master WHERE type='table' AND name='tasks'")
+        if not cursor.fetchone():
+            print("Tasks table not found. Creating new database schema...")
+            conn.close()
+            return
+
+        if check_column_exists(cursor, "tasks", "start_date"):
+            print("Column start_date already exists, skipping...")
+        else:
+            print("Adding column: start_date")
+            cursor.execute("ALTER TABLE tasks ADD COLUMN start_date DATE")
+
+        conn.commit()
+
+        cursor.execute("SELECT COUNT(*) FROM tasks")
+        task_count = cursor.fetchone()[0]
+        print(f"Migration completed successfully! {task_count} tasks unaffected (column defaults to NULL).")
+
+    except sqlite3.Error as e:
+        print(f"Database error: {e}")
+        if conn:
+            conn.rollback()
+        sys.exit(1)
+    except Exception as e:
+        print(f"Unexpected error: {e}")
+        if conn:
+            conn.rollback()
+        sys.exit(1)
+    finally:
+        if conn:
+            conn.close()
+
+
+if __name__ == "__main__":
+    print("VFX Project Management - Task Start Date Migration")
+    print("=" * 60)
+
+    migrate_database()
+
+    print("\nMigration completed successfully!")