Implement M1 authoring loop: library, editor, live preview, persistence

M1 MVP from docs/IMPLEMENTATION-PLAN.md, with the /alignment pass applied.

- core: Snippet model + factory; prepareSpecForRender (copy-not-mutate); per-theme Vega chart config
- state/orchestration: SnippetStore with debounced auto-save; IndexedDB adapter + read-time migration; startup hydration + write-through persistence
- ui: SnippetLibrary, SpecEditor (Monaco edcore.main — full editor features, JSON-only languages), LivePreview
- build: Monaco/Vega manual chunks; raised PWA precache ceiling
- alignment: flush a valid draft on snippet switch (+regression tests); TODO breadcrumbs for the preview render race and the window.confirm delete
- housekeeping: gitignore .claude/projects/

src/app/components/SpecEditor.module.css

@@ -0,0 +1,22 @@
+.editorPane {
+  position: relative;
+  height: 100%;
+}
+
+.editor {
+  height: 100%;
+  width: 100%;
+}
+
+.placeholder {
+  position: absolute;
+  inset: 0;
+  z-index: 1;
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  color: var(--color-text-muted);
+  font-size: 13px;
+  background: var(--color-bg);
+  pointer-events: none;
+}