Let themes reach icons, shape and motion (not only colours)

[CantrellJax]

Custom themes are great for colours: about 60 named roles, light and dark variants, import and preview. But a theme still can't change a few things that decide how the app feels:

1. Icons. About 300 files import from lucide-react directly, so a theme can't swap even one icon.
2. Shape and motion. Corner radius, shadows, the spacing scale and animation timing aren't theme fields, so every theme has the same shape and feel.
3. Fonts. The UI and code fonts are separate appearance settings, so a theme can't ship its own typeface.
4. A few colour spots. Some colours sit outside the roles:
   • the terminal's 16 ANSI colours, already raised in Let themes set the terminal's 16 ANSI colors #31385
   • two composer overrides keyed to data-theme-id=t3-chat

Proposal. Three small, opt-in changes, each with today's values as the defaults, so nothing changes visually unless a theme sets them:

• Icon map. One module that re-exports the icons the app uses. Components import from it instead of lucide-react, and a theme can override individual entries. This is mechanical, and splits by area.
• Shape and motion fields on ThemeDefinition: radius, shadow, spacing scale, motion duration and easing, emitted as CSS variables next to the colour roles.
• Font fields on ThemeDefinition, falling back to the current appearance settings.

The colour leaks could go through #31385 for the terminal palette, plus a tiny change to move the composer overrides onto roles.

We're building this in a downstream fork and would like it to match whatever shape you'd accept. If the direction sounds right, we can send each piece as a small PR, in whatever split and order you prefer. If you'd rather not take it, that's fine too.