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>

supabase/migrations/20260219212649_young_dragon_man.sql

@@ -0,0 +1,2 @@
+ALTER TABLE "projects" ADD COLUMN "show_scans_public" boolean DEFAULT true NOT NULL;--> statement-breakpoint
+ALTER TABLE "projects" ADD COLUMN "show_guides_public" boolean DEFAULT true NOT NULL;

supabase/migrations/meta/_journal.json

@@ -22,6 +22,13 @@
       "when": 1771478645202,
       "tag": "20260219052405_add-social-profile-fields",
       "breakpoints": true
+    },
+    {
+      "idx": 3,
+      "version": "7",
+      "when": 1771536409437,
+      "tag": "20260219212649_young_dragon_man",
+      "breakpoints": true
     }
   ]
 }