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.ts

@@ -0,0 +1,29 @@
+import { readFileSync, writeFileSync, mkdirSync } from 'fs';
+import { dirname, resolve } from 'path';
+
+export interface Preferences {
+  modelTier?: string;
+}
+
+export function loadPreferences(dataDir: string): Preferences {
+  const filePath = resolve(dataDir, 'preferences.json');
+  try {
+    const raw = readFileSync(filePath, 'utf-8');
+    return JSON.parse(raw) as Preferences;
+  } catch {
+    return {};
+  }
+}
+
+export function savePreference(dataDir: string, key: string, value: unknown): void {
+  const filePath = resolve(dataDir, 'preferences.json');
+  let prefs: Record<string, unknown> = {};
+  try {
+    prefs = JSON.parse(readFileSync(filePath, 'utf-8'));
+  } catch {
+    // start fresh
+  }
+  prefs[key] = value;
+  mkdirSync(dirname(filePath), { recursive: true });
+  writeFileSync(filePath, JSON.stringify(prefs, null, 2) + '\n');
+}