feat: add design system with colors, components, and theme configuration

- Introduced a comprehensive color palette in colors.ts, including primary, accent, success, warning, error, info, neutral, slate, and semantic colors for light and dark modes.
- Created a components-registry.ts to manage UI components with metadata, including buttons, inputs, cards, layout, feedback, and navigation components.
- Developed a theme.ts file to centralize typography, spacing, border radius, shadows, z-index, transitions, breakpoints, containers, and responsive utilities.
- Configured Nginx for development with a new nginx.dev.conf, routing API requests to the Flask backend and frontend requests to the Vite development server.

backend/app/utils/auth.py

@@ -2,7 +2,9 @@
 from flask import session
 
 TASK_ACCESS_SESSION_KEY = "task_access_ids"
+STAGED_UPLOAD_SESSION_KEY = "staged_upload_ids"
 MAX_TRACKED_TASK_IDS = 200
+MAX_TRACKED_STAGED_UPLOAD_IDS = 50
 
 
 def get_current_user_id() -> int | None:
@@ -28,14 +30,34 @@ def has_session_task_access(task_id: str) -> bool:
     return isinstance(tracked, list) and task_id in tracked
 
 
+def remember_staged_upload(upload_id: str):
+    """Persist one staged upload id in the active browser session."""
+    tracked = session.get(STAGED_UPLOAD_SESSION_KEY, [])
+    if not isinstance(tracked, list):
+        tracked = []
+
+    normalized = [value for value in tracked if isinstance(value, str) and value != upload_id]
+    normalized.append(upload_id)
+    session[STAGED_UPLOAD_SESSION_KEY] = normalized[-MAX_TRACKED_STAGED_UPLOAD_IDS:]
+
+
+def has_staged_upload_access(upload_id: str) -> bool:
+    """Return whether the active browser session owns one staged upload id."""
+    tracked = session.get(STAGED_UPLOAD_SESSION_KEY, [])
+    return isinstance(tracked, list) and upload_id in tracked
+
+
 def login_user_session(user_id: int):
     """Persist the authenticated user in the Flask session."""
     tracked_task_ids = session.get(TASK_ACCESS_SESSION_KEY, [])
+    staged_upload_ids = session.get(STAGED_UPLOAD_SESSION_KEY, [])
     session.clear()
     session.permanent = True
     session["user_id"] = user_id
     if isinstance(tracked_task_ids, list) and tracked_task_ids:
         session[TASK_ACCESS_SESSION_KEY] = tracked_task_ids[-MAX_TRACKED_TASK_IDS:]
+    if isinstance(staged_upload_ids, list) and staged_upload_ids:
+        session[STAGED_UPLOAD_SESSION_KEY] = staged_upload_ids[-MAX_TRACKED_STAGED_UPLOAD_IDS:]
 
 
 def logout_user_session():