Standardize editor and remove autosave

Created by: jedmund

Summary

• Replace autosave infrastructure with manual save (Cmd+S and save buttons)
• Add beforeunload guards to warn about unsaved changes
• Delete dead form components that imported a nonexistent Editor.svelte
• Add EditorHandle type for consistent editor instance interfaces
• Fix ProjectForm by removing broken editorRef and adding StatusDropdown with save guards

Changes

• Deleted: SimplePostForm.svelte, autosave store/controller, and related dead code (~2300 lines removed)
• Updated: Post edit/new pages to use manual save flow with Cmd+S support
• New: EditorHandle type in composer/types.ts for typed editor refs
• Updated: ProjectForm.svelte with working status dropdown and save guards

Test plan

• Verify Cmd+S saves posts correctly
• Verify unsaved changes trigger beforeunload warning
• Verify post creation and editing work without autosave
• Verify project form status changes work correctly