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;

routes/about.js

@@ -6,7 +6,7 @@ try { pkg = require('../package.json'); } catch { pkg = { version: '0.1.0' }; }
 
 router.get('/', (req, res) => {
   res.json({
-    name: 'BillTracker',
+    name: 'Bill Tracker',
     version: pkg.version,
     description: 'A self-hosted app for tracking recurring bills, monthly payments, due dates, categories, and personal bill history.',
     stack: {

services/userDbImportService.js

@@ -107,7 +107,7 @@ function normalizeName(value) {
 function parseMetadata(src) {
   const row = src.prepare("SELECT value FROM export_metadata WHERE key = 'metadata_json'").get();
   if (!row?.value) {
-    throw importError(400, 'SQLite file is not a BillTracker user data export.', 'USER_DB_IMPORT_NOT_USER_EXPORT');
+    throw importError(400, 'SQLite file is not a Bill Tracker user data export.', 'USER_DB_IMPORT_NOT_USER_EXPORT');
   }
   let metadata;
   try {
@@ -249,7 +249,7 @@ function readExportData(src) {
   const names = tableNames(src);
   const missing = REQUIRED_TABLES.filter(t => !names.has(t));
   if (missing.length) {
-    throw importError(400, 'SQLite file is not a supported BillTracker user export.', 'USER_DB_IMPORT_UNSUPPORTED_SCHEMA', missing.map(table => ({
+    throw importError(400, 'SQLite file is not a supported Bill Tracker user export.', 'USER_DB_IMPORT_UNSUPPORTED_SCHEMA', missing.map(table => ({
       table,
       message: 'Required user export table is missing',
     })));