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.

core/src/castle_core/stacks.py

@@ -520,6 +520,13 @@ def get_handler(stack: str | None) -> StackHandler | None:
     return HANDLERS.get(stack)
 
 
+def available_stacks() -> list[str]:
+    """The stack names castle has handlers for — the single source of truth for the
+    CLI ``--stack`` choices, the ``GET /stacks`` endpoint, and the dashboard select.
+    """
+    return sorted(HANDLERS)
+
+
 def _declared_commands(comp: ProgramSpec, verb: str) -> list[list[str]] | None:
     """Declared argv-lists for a verb, or None.