feat: Add template spell picker from spells.yml; polish spell table

- Add yaml-loader; import data/spells.yml at build time
- Add SpellTemplate/SpellsFile types to globals.d.ts
- Add 'Add Template Spell' button that opens a modal picker pre-filling
  all spell fields from the YAML data
- Move spell data files into data/ directory
- Split spell rows into inputs row + full-width notes row (colspan=6)
- Shrink delete column to fit-content; bold spell name input
- Add class column to spell table; change MP cost to free-text input
- Auto-resize spell notes textarea on load and on input
- Add 10px padding between spells for visual separation

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

shell.nix

@@ -0,0 +1,15 @@
+{ pkgs ? import <nixpkgs> {} }:
+
+pkgs.mkShellNoCC {
+  packages = with pkgs; [
+    go
+    gopls
+    just
+    nodejs
+    starship
+    typescript-language-server
+  ];
+  shellHook = ''
+    eval "$(starship init bash)"
+  '';
+}