feat: server-side share persistence — DB columns + API + sidebar

- pages table: added share_mode (private/invited/anyone) and published (bool)
- board.py: view_page now loads share_mode + published from DB
- page_editor.html: initializes shareTab, generalAccess, pagePublished from server
- Clipboard: fallback to textarea copy for HTTP (no HTTPS required)
- Toast: guaranteed visible with fixed positioning and Alpine x-show
- Sidebar Shared section: renders shared_pages (pages with share_mode != private)

app/db.py

@@ -138,6 +138,8 @@ def init_db():
                 content TEXT NOT NULL DEFAULT '',
                 parent_section TEXT DEFAULT 'Private',
                 parent_id INTEGER REFERENCES pages(id),
+                share_mode TEXT DEFAULT 'private',
+                published INTEGER DEFAULT 0,
                 created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
                 updated_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP
             );