feat: add shareable snippet links using base64-encoded raw URLs (Issue #23)

[amritamishra01]

Summary

This PR implements shareable snippet links for dev.lfortran.org as described in Issue #23.

Users can now:

1. Load code from any direct raw download URL (e.g. GitHub Gist raw link).
2. Click the "Share" button.
3. Generate a short link of the form:
   ?snippet=<base64_encoded_raw_url>
4. Open that link and automatically load + run the same code locally in the browser.

This follows the maintainer's vision:

Encode the download link itself using base64 — no server required.

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Implementation Details

1. Direct URL Fetching

• Supports ?url=<raw_download_link>
• Fetches the provided link directly (no URL reconstruction)
• Stores the exact download URL in sourceUrl state

2. Shareable Snippet Links

• Adds encodeSnippet and decodeSnippet utilities
• Uses URL-safe Base64 encoding
• Generates links like:
  ?snippet=<base64_encoded_raw_url>

3. Snippet Loading

• On page load, checks for ?snippet=
• Decodes the base64 string
• Fetches the decoded raw URL directly
• Loads and runs the code locally

4. UI Changes

• Added a "Share" button in TextBox
• Button is enabled only when code is loaded from an external source
• Uses Ant Design Tooltip + message for user feedback

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Architecture Notes

• Removed reliance on gist ID reconstruction.
• No GitHub-specific logic.
• No backend/server required.
• Works with any publicly accessible raw download link (e.g., GitHub Gist, repository raw file).

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Manual Testing

Tested the following:

• Load via ?url=<raw_link> → works
• Click Share → snippet link generated
• Open ?snippet=<encoded> → auto-load works
• Hard refresh → still works
• Invalid snippet → safe fallback to default example
• No 400 errors or raw URL reconstruction issues

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vid1.mp4

Closes #23