feat: propagate PL24 part metadata, improve auth UX with Google sign-in prominence

- Add unavailable/remark/modelCodes/presel fields to categories and parts DB schema
- Pass PL24 unavailable flag through pipeline instead of filtering out records
- Show unavailable categories/parts at reduced opacity in grid, tree, and parts panel
- Display part remark and model codes as secondary info in parts table
- Move Google sign-in button above email form on login/register pages with branded icon
- Add public email existence check endpoint for better login error messages
- Update INDEX.md documentation

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

apps/web/src/hooks/use-parts.ts

@@ -8,6 +8,10 @@ export interface Part {
   quantity: number | null;
   position: string | null;
   hotspotIndex: number | null;
+  unavailable?: boolean;
+  remark?: string;
+  modelCodes?: string;
+  presel?: boolean;
   price?: number;
   note?: string;
 }