feat: implement unified editor with content persistence and server-side upsert

- Replace dual update systems with single markdown-first editor architecture
- Add server-side upsert to eliminate 404 errors on PUT operations
- Fix content persistence race condition between preview and save operations
- Remove legacy updateElementContent system entirely
- Add comprehensive authentication with JWT scaffolding and dev mode
- Implement EditContext.updateOriginalContent() for proper baseline management
- Enable markdown formatting in all text elements (h1-h6, p, div, etc)
- Clean terminology: remove 'unified' references from codebase

Technical changes:
* core/editor.js: Remove legacy update system, unify content types as markdown
* ui/Editor.js: Add updateOriginalContent() method to fix save persistence
* ui/Previewer.js: Clean live preview system for all content types
* api/handlers.go: Implement UpsertContent for idempotent PUT operations
* auth/*: Complete authentication service with OAuth scaffolding
* db/queries/content.sql: Add upsert query with ON CONFLICT handling
* Schema: Remove type constraints, rely on server-side validation

Result: Clean content editing with persistent saves, no 404 errors, markdown support in all text elements

internal/auth/context.go

@@ -0,0 +1,29 @@
+package auth
+
+import "context"
+
+// Context keys for user information
+type contextKey string
+
+const (
+	userInfoContextKey contextKey = "user_info"
+)
+
+// ContextWithUser adds user information to request context
+func ContextWithUser(ctx context.Context, user *UserInfo) context.Context {
+	return context.WithValue(ctx, userInfoContextKey, user)
+}
+
+// UserFromContext extracts user information from request context
+func UserFromContext(ctx context.Context) (*UserInfo, bool) {
+	user, ok := ctx.Value(userInfoContextKey).(*UserInfo)
+	return user, ok
+}
+
+// UserIDFromContext extracts user ID from request context
+func UserIDFromContext(ctx context.Context) string {
+	if user, ok := UserFromContext(ctx); ok && user != nil {
+		return user.ID
+	}
+	return "anonymous"
+}