feat: add keyboard shortcuts for faster navigationFeat/keyboard shortcuts

[prateeknitcse]

Description

This PR adds keyboard shortcut support for common StudyPlan actions to improve accessibility and workflow speed for keyboard-first users.

The implementation is fully frontend-based and integrates with the existing UI by triggering already available button actions instead of replacing current logic.

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Features Added

Keyboard Shortcuts

Key	Action
N	Open new task modal
S	Open new subject modal
F	Open focus mode
A	Open all tasks
C	Open calendar view
/	Focus task search
Esc	Close active modal
?	Toggle shortcuts help panel

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Improvements Included

• Added searchable task filtering from the header search input
• Added keyboard shortcuts help overlay
• Prevented shortcuts while typing inside inputs/textareas/selects
• Ignored modifier combinations (Ctrl, Cmd, Alt)
• Reused existing click handlers to avoid duplicating business logic
• Added lightweight reusable keyboard UI styles (.sp-kbd)

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Files Changed

index.html

• Added global keyboard shortcut listener
• Added shortcuts help modal
• Added searchable header input
• Connected shortcuts to existing UI actions

js/app.js

• Added task search filtering logic

css/index.css

• Added keyboard shortcut helper styles
• Added utility class for modal visibility handling

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Testing Performed

• Verified all shortcuts trigger correct actions
• Tested modal open/close behavior using Esc
• Confirmed shortcuts are disabled while typing
• Verified search filtering works correctly
• Tested light/dark theme compatibility
• Checked for console errors and existing workflow regressions

Fixes #315