use core's design tokens

[pfefferle]

Proposed changes:

The app referenced 18 different --wpds-* custom properties, and 16 of them do not exist in core's design-tokens.css, so every var( --wpds-…, fallback ) silently used its fallback and nothing followed the admin theme. This maps them to the tokens core defines: dimension-padding/gap/size, typography-font-size/line-height/font-weight, border-width. Fallbacks stay, so the default theme renders the same, with three exceptions that snap to core's scale: two 16px texts become 15px, one 14px text 13px, and the sidebar focus rings go from 1.5px to core's 2px.

Values core has no token for (6px, 20px margin, 48px avatar, 64px site icon, weight 500) are plain values now instead of pretending to be tokens.

Other information:

• Have you written new tests for your changes, if applicable?

Testing instructions:

• Open the Social Web screen: it should look as before. Tab through the sidebar: focus rings are 2px.
• In dev tools, set data-wpds-corner-radius="pronounced" on <html>: cards and inspector follow the radius variant.