✨ feature: Implement Gmail message fetching and Excel sheet name truncation

tests/excel.test.ts

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+import { mkdtemp, rm } from 'node:fs/promises';
+import { tmpdir } from 'node:os';
+import { join } from 'node:path';
+import { afterEach, beforeEach, describe, expect, it } from 'vitest';
+import XLSX from 'xlsx';
+import { ExcelWriter } from '../src/output/excel.js';
+
+let dir = '';
+
+beforeEach(async () => {
+  dir = await mkdtemp(join(tmpdir(), 'nlc-excel-'));
+});
+
+afterEach(async () => {
+  await rm(dir, { force: true, recursive: true });
+});
+
+describe('ExcelWriter', () => {
+  it('truncates newsletter sheet names to the Excel 31-character limit', async () => {
+    const path = join(dir, 'catalog.xlsx');
+    const newsletter = 'A Very Long Newsletter Name That Exceeds The Excel Limit';
+
+    await new ExcelWriter(path).write({
+      rows: [
+        {
+          'Source Newsletter': newsletter,
+          Title: 'Post',
+          'Link URL': 'https://example.com'
+        }
+      ],
+      sponsors: [],
+      deadLinks: []
+    });
+
+    const workbook = XLSX.readFile(path);
+    expect(workbook.SheetNames[0]).toBe('A Very Long Newsletter Name Tha');
+    expect(workbook.SheetNames[0].length).toBe(31);
+  });
+});

tests/gmail.test.ts

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+import { describe, expect, it } from 'vitest';
+import { buildBrowserCommand, GmailClient } from '../src/gmail/client.js';
+
+describe('GmailClient', () => {
+  it('uses PowerShell to open Windows OAuth URLs without splitting query parameters', () => {
+    const url =
+      'https://accounts.google.com/o/oauth2/v2/auth?access_type=offline&response_type=code';
+
+    expect(buildBrowserCommand(url, 'win32')).toEqual({
+      file: 'powershell.exe',
+      args: ['-NoProfile', '-Command', 'Start-Process -FilePath $args[0]', url]
+    });
+  });
+
+  it('loads HTML messages from the configured Gmail label', async () => {
+    const calls: string[] = [];
+    const gmail = {
+      users: {
+        labels: {
+          list: async () => ({
+            data: { labels: [{ id: 'Label_1', name: 'Newsletters' }] }
+          })
+        },
+        messages: {
+          list: async (_params: unknown) => {
+            calls.push('list');
+            return { data: { messages: [{ id: 'msg-1' }] } };
+          },
+          get: async () => {
+            calls.push('get');
+            return {
+              data: {
+                id: 'msg-1',
+                payload: {
+                  headers: [
+                    { name: 'Message-ID', value: '<msg-1@example.com>' },
+                    { name: 'From', value: 'Weekly <weekly@example.com>' },
+                    { name: 'Date', value: 'Sat, 16 May 2026 10:00:00 -0500' }
+                  ],
+                  parts: [
+                    {
+                      mimeType: 'text/html',
+                      body: {
+                        data: Buffer.from(
+                          '<h2>Python</h2><a href="https://example.com">Post</a>'
+                        ).toString('base64url')
+                      }
+                    }
+                  ]
+                }
+              }
+            };
+          }
+        }
+      }
+    };
+
+    const messages = await new GmailClient(gmail).fetchMessages('Newsletters', { maxResults: 5 });
+
+    expect(calls).toEqual(['list', 'get']);
+    expect(messages).toEqual([
+      {
+        id: 'msg-1',
+        messageId: '<msg-1@example.com>',
+        from: 'Weekly <weekly@example.com>',
+        date: '2026-05-16T15:00:00.000Z',
+        subject: '',
+        html: '<h2>Python</h2><a href="https://example.com">Post</a>',
+        headers: {
+          date: 'Sat, 16 May 2026 10:00:00 -0500',
+          from: 'Weekly <weekly@example.com>',
+          listId: undefined,
+          messageId: '<msg-1@example.com>',
+          subject: ''
+        }
+      }
+    ]);
+  });
+});