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.

cli/src/castle_cli/main.py

@@ -12,6 +12,8 @@ from __future__ import annotations
 import argparse
 import sys
 
+from castle_core.stacks import available_stacks
+
 from castle_cli import __version__
 
 DEV_VERBS = ["build", "test", "lint", "format", "type-check", "check"]
@@ -37,11 +39,7 @@ def _build_program_group(subparsers: argparse._SubParsersAction) -> None:
 
     p = sub.add_parser("create", help="Scaffold a new program")
     _add_name(p, "Program name")
-    p.add_argument(
-        "--stack",
-        choices=["python-cli", "python-fastapi", "react-vite", "supabase"],
-        default=None,
-    )
+    p.add_argument("--stack", choices=available_stacks(), default=None)
     p.add_argument("--description", default="", help="Program description")
     p.add_argument("--port", type=int, help="Port (daemons only)")