Populate the program stack select from castle-api

The dashboard's stack dropdown hardcoded its options (python-cli/fastapi/
react-vite), so a supabase program showed "(none)" — the value had no matching
option. Make the backend the single source of truth instead:

- core: available_stacks() returns sorted(HANDLERS) — the stacks castle has
  handlers for.
- castle-api: GET /stacks exposes it.
- cli: --stack choices derive from available_stacks() (drops the duplicated list).
- app: ProgramFields fetches /stacks (useStacks) and renders options from it,
  always including the current value so it never silently blanks; StackBadge
  gets a Supabase label.

A new backend stack now appears everywhere without a frontend edit.

app/src/lib/labels.ts

@@ -22,6 +22,7 @@ export const STACK_LABELS: Record<string, string> = {
   "python-fastapi": "Python / FastAPI",
   "python-cli": "Python / CLI",
   "react-vite": "React / Vite",
+  supabase: "Supabase",
   rust: "Rust",
   go: "Go",
   bash: "Bash",