feat: add dark/light mode toggle with persistent theme support

[pari-dubey1]

Summary [required]

This PR adds a fully functional dark/light mode toggle feature to improve accessibility and overall user experience across the DevPath website. Users can now switch between light and dark themes dynamically, with the selected preference being saved using localStorage so the theme persists across refreshes and future visits. The update also improves responsive behavior by adding a dedicated mobile theme toggle inside the hamburger navigation menu and applies dark mode styling consistently across major homepage sections, cards, forms, buttons, navigation, footer, and interactive UI components.

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Related Issue [required]

Closes #283

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Type of Change [required]

• Bug fix — resolves a broken behaviour
• Feature — adds new functionality
• Data — adds new projects to data/projects.json
• Documentation — updates docs, README, or code comments only
• Style — CSS or visual changes only, no logic change
• Refactor — restructures code without changing behaviour
• Test — adds or updates tests

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What Was Changed [required]

File	Change made
static/style.css	Added complete dark mode styling for navbar, hero, cards, sections, footer, forms, buttons, and responsive components
static/script.js	Added theme toggle logic, localStorage persistence, theme switching functionality, and mobile theme synchronization
templates/index.html	Added desktop and mobile theme toggle buttons inside navbar and hamburger menu

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How to Test This PR [required]

1. Clone this branch:

git checkout feat/dark-mode-toggle

2. Install dependencies:

pip install -r requirements.txt

3. Run the application:

python app.py

4. Open:

http://127.0.0.1:5000

5. Verify the following:

• Theme toggle button works on desktop
• Mobile hamburger menu contains working theme toggle
• Theme persists after refresh
• Dark mode styles apply correctly across all sections
• Navigation, cards, buttons, forms, and footer remain readable
• Responsive layout works correctly in both themes

6. Run tests:

python tests/test_basic.py

Expected test output:

27 passed, 0 failed out of 27 tests

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Test Results [required]

27 passed, 0 failed out of 27 tests

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Screenrecording

Screen.Recording.2026-05-22.160504.mp4

Screen.Recording.2026-05-22.160559.mp4

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Self-Review Checklist [required]

• I have read CONTRIBUTING.md and followed all guidelines
• My branch name follows the convention: feat/, fix/, docs/, data/, style/, test/
• I have run python tests/test_basic.py and all 27 tests pass
• I have run flake8 . locally and there are no errors
• I have not introduced any print() or console.log() debug statements
• Every new function I wrote has a docstring
• I have not modified files outside the scope of the linked issue
• If I changed the UI, I tested it at 375px (mobile) and 1280px (desktop)
• If I added a project to the dataset, it has all required JSON fields

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Notes for Reviewer

• Added both desktop and mobile theme toggle support
• Theme preference is persisted using localStorage
• Verified responsiveness for both light and dark themes
• Adjusted dark mode styles for readability and accessibility across all homepage sections

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