feat: add project visibility settings for scans and floorplans (#106)

- Add show_scans_public and show_guides_public columns to projects table
- Add visibility toggles to editor sidebar settings panel (auto-save)
- Add visibility toggles to project settings dialog (auto-save)
- Propagate settings to community viewer (hide scans/guides for non-owners)
- Add updateProjectVisibility server action
- Fix username onboarding dialog stuck after DB reset (validate update affected rows)
- Add error handling to UsernameGate for stale sessions

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

packages/db/src/schema/projects/projects.ts

@@ -20,6 +20,8 @@ export const projects = pgTable(
     metadata: t.jsonb('metadata'),
     // Community features
     isPrivate: t.boolean('is_private').notNull().default(true),
+    showScansPublic: t.boolean('show_scans_public').notNull().default(true),
+    showGuidesPublic: t.boolean('show_guides_public').notNull().default(true),
     views: t.integer('views').notNull().default(0),
     likes: t.integer('likes').notNull().default(0),
     thumbnailUrl: t.text('thumbnail_url'),