refactor: enhance TimerPage layout and functionality; update theme and tokens for improved design consistency

- Refactored TimerPage component to improve layout and user experience.
- Updated theme configuration to utilize design tokens for better maintainability.
- Introduced new tokens for colors and typography to align with design principles.
- Simplified state management and improved timer functionality.
- Enhanced accessibility and responsiveness of UI elements.

backend/app/config.py

@@ -16,15 +16,20 @@ ever opening the settings page, and anything you later change in the UI wins.
 
 from __future__ import annotations
 
+import os
 from functools import lru_cache
 
 from pydantic import Field
 from pydantic_settings import BaseSettings, SettingsConfigDict
 
+# Tests set this so a developer's real `.env` — with live Jira and Telegram
+# credentials in it — can never leak into a test run.
+_DOTENV_FILES = () if os.environ.get("KADENCE_NO_DOTENV") else (".env", "../.env")
+
 
 class EnvSettings(BaseSettings):
     model_config = SettingsConfigDict(
-        env_file=(".env", "../.env"),
+        env_file=_DOTENV_FILES,
         env_file_encoding="utf-8",
         extra="ignore",
     )