Align shadcn's accent token with what shadcn means by it

shadcn uses `bg-accent` for its SUBTLE surfaces — dropdown item hover, command
row selection, ghost and outline button hover, the dialog close affordance. The
brand colour in shadcn is `primary`.

PIG's Tailwind config mapped `accent` to `--accent`, which is the brand. That
inverted the meaning, so every shadcn hover and selection state painted a
full-strength brand block. With the monochrome "pig" palette in dark mode the
brand is near-white, so a selected command row rendered as a white slab against
a near-black sheet. Measured before the change: selected row rgb(250,250,250)
on a rgb(9,9,11) body.

`accent` now aliases `--accent-subtle` and `accent-foreground` aliases
`--accent-fg`, which is what those tokens were created for. The eleven places
where PIG's own components wanted a solid brand fill — filled chips, selected
card borders, progress bars — move to `primary`, which still resolves to
`--accent`. A `brand` alias is added for clarity.

After: selected row rgb(39,39,42) in dark and rgb(244,244,245) in light, both a
subtle tint above the body; the pipeline's active stage chip stays a solid
rgb(250,250,250) fill, unchanged.

Found by opening overlays, which earlier screenshot sweeps never did — every
route had been checked, but a dropdown or a command palette only misbehaves
once it is open. Worth remembering: page-level sweeps do not exercise portals.

Typecheck clean, 135 unit tests and e2e green, CSP hash unchanged, 0px
horizontal overflow across 12 routes at 393px and 1440px.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

apps/web/src/components/PiggyChat.tsx

@@ -264,7 +264,7 @@ function PiggyChatPanel({
 
 function ChatMessage({ message }: { message: TranscriptMessage }) {
   if (message.role === 'user') {
-    return <div className="ml-auto max-w-[88%] rounded-2xl rounded-br-md bg-accent px-4 py-3 text-sm text-accent-on"><p className="whitespace-pre-wrap">{message.content}</p></div>;
+    return <div className="ml-auto max-w-[88%] rounded-2xl rounded-br-md bg-primary px-4 py-3 text-sm text-accent-on"><p className="whitespace-pre-wrap">{message.content}</p></div>;
   }
   return (
     <div className="flex gap-3">