feat(tui): persist model tier selection and fix formatting

Persist /model tier choice to ~/.local/share/flynn/preferences.json so
it survives restarts. Decode HTML entities (e.g. ') in markdown
renderer output. Suppress noisy logger.info and punycode deprecation
warnings in TUI startup.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/preferences.test.ts

@@ -0,0 +1,46 @@
+import { describe, it, expect, beforeEach, afterEach } from 'vitest';
+import { mkdtempSync, rmSync, readFileSync, writeFileSync, mkdirSync } from 'fs';
+import { resolve } from 'path';
+import { tmpdir } from 'os';
+import { loadPreferences, savePreference } from './preferences.js';
+
+describe('preferences', () => {
+  let dataDir: string;
+
+  beforeEach(() => {
+    dataDir = mkdtempSync(resolve(tmpdir(), 'flynn-prefs-'));
+  });
+
+  afterEach(() => {
+    rmSync(dataDir, { recursive: true, force: true });
+  });
+
+  it('returns empty object when file is missing', () => {
+    expect(loadPreferences(dataDir)).toEqual({});
+  });
+
+  it('returns empty object when file is corrupt', () => {
+    writeFileSync(resolve(dataDir, 'preferences.json'), 'not json!!!');
+    expect(loadPreferences(dataDir)).toEqual({});
+  });
+
+  it('round-trips a saved preference', () => {
+    savePreference(dataDir, 'modelTier', 'local');
+    const prefs = loadPreferences(dataDir);
+    expect(prefs.modelTier).toBe('local');
+  });
+
+  it('merges preferences without overwriting other keys', () => {
+    savePreference(dataDir, 'modelTier', 'fast');
+    savePreference(dataDir, 'otherKey', 42);
+    const raw = JSON.parse(readFileSync(resolve(dataDir, 'preferences.json'), 'utf-8'));
+    expect(raw.modelTier).toBe('fast');
+    expect(raw.otherKey).toBe(42);
+  });
+
+  it('creates parent directories if needed', () => {
+    const nested = resolve(dataDir, 'sub', 'dir');
+    savePreference(nested, 'modelTier', 'default');
+    expect(loadPreferences(nested).modelTier).toBe('default');
+  });
+});