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

db/sqlite/schema.sql

@@ -4,7 +4,7 @@ CREATE TABLE content (
     id TEXT NOT NULL,
     site_id TEXT NOT NULL,
     value TEXT NOT NULL,
-    type TEXT NOT NULL CHECK (type IN ('text', 'markdown', 'link')),
+    type TEXT NOT NULL,
     created_at INTEGER DEFAULT (strftime('%s', 'now')) NOT NULL,
     updated_at INTEGER DEFAULT (strftime('%s', 'now')) NOT NULL,
     last_edited_by TEXT DEFAULT 'system' NOT NULL,