feat: Add dark mode toggle option

[AgentQuantum]

Add Dark Mode Support (feature/dark-mode)

What

• Introduces a new Dark Mode theme option to the application UI.
• Adds a toggle (or configuration option) for Dark Mode in the settings/menu.
• Implements global dark-theme CSS variables and overrides existing styles so that major UI components (sidebar, navigation, cards, forms, tables, modals, etc.) render appropriately in Dark Mode.
• Ensures that Light Mode remains unchanged by default.

How

• Defined a set of CSS custom properties (variables) for color palette (backgrounds, text, borders, hover states, etc.) for both light and dark themes.
• Modified the base styles and component styles to reference these variables rather than hard-coded colors.
• Added logic (e.g. in configuration or user settings) to switch between themes — updating a root class or toggling a theme attribute so CSS variables take effect accordingly.
• Updated UI components and layout as needed to maintain readability and layout integrity under dark background (e.g. adjusting contrast, hover/focus states, backgrounds, shadows).
• (Optional / if implemented) Persist user preference of theme (e.g. in local storage or config) so the selected mode sticks between sessions.

Why

• Many users prefer Dark Mode for comfort, especially when using the app in low-light or nighttime conditions — reduces eye strain.
• Provides better usability and accessibility for users sensitive to bright themes or working long hours.
• Adds theming support, making the UI more flexible and future-proof for additional themes or user-customizable styles.
• Aligns with modern UX expectations; many tools and apps offer Dark Mode as a standard feature.

Acceptance Criteria

• A “Dark Mode” toggle or setting appears in the configuration/settings menu.
• When Dark Mode is enabled, all major UI sections (sidebar, nav, cards/panels, tables/lists, forms, modals) render with dark backgrounds and correct text visibility (sufficient contrast, no unreadable text).
• When toggling back to Light Mode, the UI returns to the original light theme exactly as before — no residual dark-mode styles leak.
• The theme change applies globally (not only to a subset of pages/components).
• The selection of theme persists across page reloads (or application restarts), if persistence implemented.
• No visual regressions in layout, spacing, or functionality — UI components (buttons, inputs, hover/focus states, modals, dialogs) behave and render correctly in both themes.
• Styles for any third-party or external UI components/widgets are reviewed to confirm they do not break in Dark Mode (or are gracefully handled/fallback if unsupported).

Closes #46