[Feature]: Add GSSoC 2025 and 2026 program details — the app currently only covers GSSoC 2024

## Summary

OpSo is designed to be a comprehensive information hub for open-source programs. The README and Open Source Programs section acknowledge that OpSo is itself part of GSSoC, yet the app appears to contain data only for GSSoC 2024. GSSoC 2025 concluded and GSSoC 2026 is actively ongoing — thousands of contributors using this exact app during GSSoC 2026 cannot find information about the current edition they are participating in.

## Problem

- The app's program data for GSSoC stops at the 2024 edition (visible in the `assets/` folder and `lib/` data files).
- New contributors joining GSSoC 2026 who install OpSo will find outdated information — wrong dates, old mentors, stale eligibility criteria.
- This is especially ironic given that OpSo is itself a GSSoC project.
- The same gap likely exists for other programs that had 2025/2026 editions (GSoC 2025, MLH Fellowship, etc.).

## Proposed Solution

I would like to add complete data cards for:

1. **GSSoC 2026** — current edition with accurate dates, eligibility, how-to-apply, and mentor resources
2. **GSSoC 2025** — retroactive addition for completeness
3. **GSoC 2025** — update the Google Summer of Code section with 2025 results and 2026 application data

**For each program update, I will:**

- Create or update the data file in `lib/` (following the existing data model for the program screen)
- Create or update the corresponding Flutter screen in `lib/` (following the existing screen pattern)
- Add/update the entry in the home screen program list
- Add any new logo assets to `assets/`

I'll follow the exact same code structure as the existing GSSoC 2024 implementation so the PR is clean and easy to review.

Please assign this to me!

**Labels:** `enhancement`, `content`, `help wanted`, `GSSoC 2026`

Inspect the existing GSSoC 2024 data files and corresponding Flutter screen in lib/, then review the home-screen program list and related assets/. Follow that structure for the GSSoC 2025 and 2026 details and the GSoC 2025/2026 updates. Done means the requested program information, screens, list entries, and logos are present and accurate.

由索引模型根據 Issue 內容生成。