Add shareable DisplayList encoding URLs - #4

Summary

• encode the canonical minimalDescription as versioned UTF-8 Base64URL in the URL fragment
• add a Copy link action and restore shared links in reverse-conversion mode
• keep URL state synchronized without sending DisplayList data in HTTP requests
• add URL codec coverage and run it in CI and Pages builds

Why

Sharing the original DisplayList.description produces unnecessarily long URLs and preserves details that are not needed to inspect the encoding. In the checked fixture, the source description is 3,505 bytes while its canonical minimalDescription is 91 bytes. The compact form retains the encoding structure and makes reconstruction loss explicit with the existing ? and * placeholders.

User impact

Users can copy a self-contained link, open it in another browser, and immediately inspect the same encoding. The URL fragment stays browser-local and is removed when the editor is cleared or becomes invalid.

Validation

• swift test — 11 tests passed
• npm test — 4 tests passed
• SwiftWasm release build
• Vite Pages production build
• browser verification of copy, reload/decode, reverse mode, and clear behavior
• preview endpoint returned HTTP 200