feat: Add Enter key navigation for form inputs (Feature #13)

[adithyen]

Pull Request: Feature #13 – Enter Key Navigation & Submission

Linked Issue

Closes #13

Description

Adds keyboard-first form flow to the Expense Tracker. Users can now fill out and submit the Add Expense form entirely with the keyboard — no mouse clicks required.

Changes Made

script.js

• Added setupEnterKeyNavigation() function that attaches keydown listeners to all form inputs
• Tab-like Enter navigation: Name → Amount → Date → Category → submit
• Category (select): Enter triggers addExpense()
• Monthly Budget: Enter triggers setBudget()
• Search field: Enter triggers applyFilters()
• All handlers use e.preventDefault() to suppress default browser behaviour

index.html

• Added jsPDF CDN libraries (from previous session — included in this commit)
• Updated dark mode button to emoji style 🌙/☀️
• Added 📄 Export to PDF button
• Added confirm modal

style.css

• Added styles for .dark-mode-btn, .export-btn, .modal, .modal-content (from previous session)

How to Test

1. Open index.html (with backend running via node backend/server.js)
2. Click Expense Name and type a name
3. Press Enter → focus moves to Amount
4. Type an amount, press Enter → focus moves to Date
5. Enter a date, press Enter → focus moves to Category
6. Press Enter → expense is submitted (same as clicking Add)
7. Click Monthly Budget, type a value, press Enter → budget is set
8. Click Search, type something, press Enter → filters apply

##Screenshots

Checklist

• Code follows existing style conventions
• No double-submit risk (e.preventDefault() on all handlers)
• All element IDs verified to exist in [index.html]
• Works alongside existing onclick button handlers