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/db/sqlite/content.sql.go

@@ -212,3 +212,42 @@ func (q *Queries) UpdateContent(ctx context.Context, arg UpdateContentParams) (C
 	)
 	return i, err
 }
+
+const upsertContent = `-- name: UpsertContent :one
+INSERT INTO content (id, site_id, value, type, last_edited_by)
+VALUES (?1, ?2, ?3, ?4, ?5)
+ON CONFLICT(id, site_id) DO UPDATE SET
+    value = EXCLUDED.value,
+    type = EXCLUDED.type,
+    last_edited_by = EXCLUDED.last_edited_by
+RETURNING id, site_id, value, type, created_at, updated_at, last_edited_by
+`
+
+type UpsertContentParams struct {
+	ID           string `json:"id"`
+	SiteID       string `json:"site_id"`
+	Value        string `json:"value"`
+	Type         string `json:"type"`
+	LastEditedBy string `json:"last_edited_by"`
+}
+
+func (q *Queries) UpsertContent(ctx context.Context, arg UpsertContentParams) (Content, error) {
+	row := q.db.QueryRowContext(ctx, upsertContent,
+		arg.ID,
+		arg.SiteID,
+		arg.Value,
+		arg.Type,
+		arg.LastEditedBy,
+	)
+	var i Content
+	err := row.Scan(
+		&i.ID,
+		&i.SiteID,
+		&i.Value,
+		&i.Type,
+		&i.CreatedAt,
+		&i.UpdatedAt,
+		&i.LastEditedBy,
+	)
+	return i, err
+}

internal/db/sqlite/querier.go

@@ -22,6 +22,7 @@ type Querier interface {
 	InitializeSchema(ctx context.Context) error
 	InitializeVersionsTable(ctx context.Context) error
 	UpdateContent(ctx context.Context, arg UpdateContentParams) (Content, error)
+	UpsertContent(ctx context.Context, arg UpsertContentParams) (Content, error)
 }
 
 var _ Querier = (*Queries)(nil)