fix(editor): restore mobile Save button outline and fix dark toast text contrast

[bouzidanas]

What & why

This PR fixes four small editor UI issues:

1. Mobile Save button outline

   Below 700px, the save-as dropdown caret element is hidden, but Save retained the styling making it still look like the left half of a split button. This PR restores the missing right border and corner radii so Save renders as a complete button.

2. Toast text contrast

   In dark mode, toast backgrounds switch through the theme while their fixed white text did not. Toast text now uses var(--surface) so it remains readable in both themes.

3. Primary button text contrast

   .ed-btn-primary uses var(--ink) as its background. Since --ink changes from dark in light mode to light in dark mode, a fixed white text color becomes unreadable in dark mode. Its text now uses var(--surface).

4. Active set-chip text contrast

   .ed-setchip.active has the same inverted theme treatment and the same fixed-white-text issue. Its text now also uses var(--surface).

UI comparisons

Mobile Save button

Toast

Primary Share action

Active hover-set chip

Verification

• Typecheck passed
• npm run build:single passed
• Browser-tested the folded topbar at 390px
• Browser-tested the toast in light and dark modes
• Browser-tested the primary Share action in dark mode
• Browser-tested the active hover-set chip in dark mode

Checklist

• Read the relevant repository guidance before making changes
• npm run build:single succeeds from slides/
• No sync code was changed
• No new UI strings were added
• No document-format changes were made
• Did not bump the version or cut a release

[bouzidanas]

There is a similar issue (as the toast text color) with the button in the toast/info dialog that appears at the top when editor is opened in Safari browser in dark mode.

Edit: latest commit fixes this now.