bug: add social media icons and profile fallback (JhaSourav07#3034)

## Description

This PR implements the missing social media sharing icons and adds a
robust profile picture fallback (PFB) system using user initials when a
GitHub avatar is unavailable.

Fixes JhaSourav07#3020 

## Pillar
- [ ] 🎨 Pillar 1 — New Theme Design
- [x] 📐 Pillar 2 — Geometric SVG Improvement
- [ ] 🕐 Pillar 3 — Timezone Logic Optimization
- [ ] 🛠️ Other (Bug fix, refactoring, docs)

## Visual Preview
- Before
<img width="543" height="830" alt="image"
src="https://github.com/user-attachments/assets/cc379d29-d8bf-41c7-a2bb-1c8d4f407e25"
/>

- After
<img width="498" height="766" alt="image"
src="https://github.com/user-attachments/assets/f2bf9e2a-7fb4-46f0-9535-3db76209a7ea"
/>

## Checklist before requesting a review:

- [x] I have read the `CONTRIBUTING.md` file.
- [x] I have tested these changes locally
(`localhost:3000/api/streak?user=YOUR_USERNAME`).
- [x] I have run `npm run format` and `npm run lint` locally and
resolved all errors[cite: 2].
- [x] I have run `npm run test` and all tests pass locally[cite: 2].
- [x] I have run `npm run test:coverage` and branch coverage is at or
above 70%[cite: 2].
- [x] My commits follow the Conventional Commits format (e.g.,
`feat(ui): ...`)[cite: 2].
- [x] I have updated `README.md` if I added a new theme or URL parameter
(if applicable).
- [x] I have started the repo.
- [x] I have made sure that i have only one commit to merge in this PR.
- [x] The SVG output matches the CommitPulse "premium quality" aesthetic
standard[cite: 2].
- [x] (Recommended) I joined the CommitPulse Discord server for faster
collaboration, mentorship, and PR support[cite: 2].

Author: JhaSourav07