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/api/src/categories/categories.service.ts

@@ -250,6 +250,7 @@ export class CategoriesService {
         externalId: sg.code,
         linkPath: sg.linkPath || null,
         linkWid: sg.linkWid || null,
+        unavailable: sg.unavailable || false,
         source: "pl24" as const,
       }));
 
@@ -435,6 +436,10 @@ export class CategoriesService {
                   const val = parseInt(p.hotspotId!, 10);
                   return (val > 0 && val <= 2147483647) ? val : null;
                 })() : null,
+                unavailable: p.unavailable || false,
+                remark: p.remark || null,
+                modelCodes: p.modelCodes || null,
+                presel: p.presel || false,
                 source: "pl24" as const,
               }));