feat: Add each new link to browser history

[konard]

🎯 Summary

Implements browser history support for the Link Calculator, allowing users to navigate between different calculations using the browser's back and forward buttons.

📋 Issue Reference

Fixes #39

🔧 Implementation Details

Changes to useUrlExpression hook:

• Modified URL synchronization to use pushState instead of replaceState for subsequent expression changes
• First URL update (on initial load) still uses replaceState to avoid creating unnecessary history entries
• Added popstate event listener to handle browser back/forward navigation
• When user navigates back/forward, the expression input is automatically updated from the URL

Testing:

• Added comprehensive E2E test that verifies:
  • Multiple expressions create separate history entries
  • Browser back button navigates to previous expressions
  • Browser forward button navigates to next expressions
  • Input and results update correctly during navigation

📝 Technical Approach

The implementation uses a useRef flag to track whether the current update is the first one after component mount. This ensures:

1. Initial page load uses replaceState (no duplicate history entry)
2. All user-initiated changes use pushState (creates new history entries)
3. Browser navigation works seamlessly with the popstate handler

✅ Testing

• Unit tests: All existing tests pass (encode/decode functions unchanged)
• E2E tests: New test verifies complete browser history workflow
• Manual testing: Verified back/forward navigation works correctly

📦 Files Changed

• web/src/hooks/useUrlExpression.ts - Modified hook to use pushState and handle popstate
• web/e2e/calculator.spec.ts - Added E2E test for browser history
• changelog.d/20260126_155726_browser_history.md - Changelog fragment

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This PR was created with assistance from Claude Code

[konard]

🤖 Solution Draft Log

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💰 Cost estimation:

• Public pricing estimate: $2.132965 USD
• Calculated by Anthropic: $1.256363 USD
• Difference: $-0.876603 (-41.10%)
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