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/database/schema/core.ts

@@ -274,6 +274,7 @@ export const categories = pgTable(
     externalId: varchar("external_id", { length: 100 }),
     linkPath: text("link_path"),
     linkWid: varchar("link_wid", { length: 100 }),
+    unavailable: boolean("unavailable").default(false).notNull(),
     source: varchar("source", { length: 20 }).default("pl24").notNull(),
     createdAt: timestamp("created_at", { withTimezone: true }).defaultNow().notNull(),
   },
@@ -320,6 +321,10 @@ export const parts = pgTable(
     quantity: integer("quantity"),
     position: varchar("position", { length: 100 }),
     hotspotIndex: integer("hotspot_index"),
+    unavailable: boolean("unavailable").default(false).notNull(),
+    remark: text("remark"),
+    modelCodes: varchar("model_codes", { length: 500 }),
+    presel: boolean("presel").default(false).notNull(),
     source: varchar("source", { length: 20 }).default("pl24").notNull(),
     createdAt: timestamp("created_at", { withTimezone: true }).defaultNow().notNull(),
   },