feat: use URL-safe base64url encoding for shareable links

[konard]

Summary

• Replace standard base64 URL encoding with URL-safe base64url encoding (RFC 4648 Section 5)
• URLs now use only a-zA-Z0-9, -, and _ characters - no more ugly URL encoding like %3D for =
• Auto-calculate when expression is loaded from URL (calculation triggered immediately)
• Backward compatibility: old base64 URLs are automatically redirected to new format
• Prevent duplicate browser history entries when same expression is recalculated

Technical Details

The new encoding:

• Converts + to -
• Converts / to _
• Removes padding =

This makes URLs shorter and cleaner while maintaining full backward compatibility with existing shared links.

Test plan

• Run unit tests (npm test in web/)
• Run Rust tests (cargo test)
• Run linting checks (cargo fmt --check && cargo clippy)
• Verify new encoding produces only URL-safe characters
• Verify old base64 URLs still decode correctly
• Verify auto-redirect from old format to new format
• Verify calculation triggers automatically on URL load
• Verify no duplicate history entries on recalculation

Fixes #49

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[konard]

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