fix(brand): finish the "BillTracker" → "Bill Tracker" rename (display copy)

The v0.42.0 brand refresh renamed client copy but missed a few user-facing
server strings:
- routes/about.js: the /api/about `name` (rendered as the About page title) was
  still 'BillTracker' → 'Bill Tracker'.
- userDbImportService.js: two import-error messages referenced 'BillTracker'.
- Remove the now-unused APP_NAME export from lib/version.ts (0 consumers;
  lib/brand.ts BRAND.name is the canonical source).

Left intentionally as identifiers (not display copy): the updateCheckService
User-Agent 'BillTracker/<version>' and all repository URLs.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

client/lib/version.ts

@@ -3,7 +3,6 @@
 declare const __APP_VERSION__: string;
 
 export const APP_VERSION = __APP_VERSION__;
-export const APP_NAME    = 'Bill Tracker';
 
 export interface ReleaseHighlight {
   icon: string;