picker: encode selected color in URL so shared links work

[Aditya30ag]

Closes #52

Problem

The picker only encoded color spaces in the URL (/picker/oklch+p3), never
the color value. Sharing a link always opened the default lch(50 50 50)
for the recipient regardless of what was selected.

Changes

picker/index.js

• data() reads ?color= on load, with priority over localStorage
• updateLocation() includes the color in its pushState call
• New updateColorInURL() uses replaceState on every color change URL stays shareable without spamming browser history on every slider tick

Before / After

Before: /picker/oklch
After:  /picker/oklch?color=oklch(72%25%200.18%20142)

Opening the "after" URL in a other window now restores both the color space and the exact color.

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