FE 029: Add employer keyboard shortcuts

[Deepanshugarg11]

Here is a short, concise version of the PR description:

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PR: Add Keyboard Shortcuts for Employer Panel (FE 029)

Summary

Adds global keyboard shortcuts and a shortcut help modal to the Employer Panel.

⌨️ Shortcuts Added

Shortcut	Action
Ctrl/Cmd + Shift + S	Open Shifts (/manage-shift)
Ctrl/Cmd + Shift + G	Open Guards (/guard-profiles)
Ctrl/Cmd + Shift + P	Open Payroll (/payroll)
N	Create New Shift (/create-shift)
R	Refresh Page
?	Open Keyboard Shortcuts Modal
Esc	Close Shortcuts Modal

Key Highlights

• Form Field Bypassing: Shortcuts automatically ignore input when typing in <input>, <textarea>, <select>, or editable fields.
• OS & Browser Safe: Supports both Windows/Linux (Ctrl) and Mac (Cmd). Normal browser shortcuts (e.g. Ctrl+C, Ctrl+V, Ctrl+R) are not affected.
• Zero Dependencies: Pure React & Vanilla CSS matching SecureShift branding.

🧪 Testing

• Unit Tests: All 27 tests passed (npm test).
• Linting: 0 errors/warnings on new files.