Description

This pull request adds a global keyboard shortcut (Alt + T on Windows/Linux and Option + T on macOS) to toggle the dark/light theme across all application pages (including the dashboard, settings, and public profile pages). It also updates the Shortcuts panel to dynamically reflect the correct keys depending on the client OS.

Problem

Previously, the theme toggle shortcut triggered on the letter T alone (prone to accidental triggers) and was only active on the Dashboard page because the listener was coupled with the Dashboard Header component. On the settings or public profile pages, it did not work. Additionally, the shortcuts overlay listed the key as "T".

Solution

1. Created GlobalKeyboardShortcuts.tsx: A new root-level, headless keyboard shortcut listener component mounted inside Providers.tsx. It handles the global key combinations and includes screen-reader accessibility announcements (aria-live="polite").
2. OS Auto-Detection: Integrated dynamic platform detection in ShortcutsModal.tsx using navigator.userAgent to display Option + T on Mac platforms and Alt + T on other operating systems.
3. Decoupled UI & Logic: Simplified KeyboardShortcuts.tsx into a lightweight trigger button that dispatches a custom window event (openShortcuts) to open the global shortcuts modal.
4. Input Avoidance: Guaranteed that shortcut hotkeys do not fire when a user has focus inside an input, textarea, select, or contenteditable container.

Acceptance Criteria Checklist

• Alt+T / Option+T toggles theme globally.
• Works seamlessly on the dashboard, settings, and public profile pages.
• Does not fire when the user is typing in an input, textarea, select, or contenteditable.
• Shortcut listed in the keyboard shortcuts overlay panel as Option + T (on Mac) or Alt + T (on Windows/Linux).

How to Test

1. Press Alt+T (or Option+T on Mac) on the dashboard, settings page, and public profile page to verify the theme toggles globally.
2. Select any input or textarea on the settings page and type words with 't' and 'Alt+t'. Confirm the theme does not toggle while typing.
3. Click the "Shortcuts" button or press ? on the keyboard to open the Shortcuts modal.
4. Check that the label for "Toggle theme" dynamically displays Option + T on macOS devices and Alt + T on Windows/Linux devices.