chore: v0.4.0 — bump version + CHANGELOG entry

manifest.json version 0.3.0 → 0.4.0. CHANGELOG.md entry for v0.4.0
documents the first-run UX hint feature (merged in b82f14e via
v0.4.0-prep branch). Zero new permissions, zero new dependencies,
zero behaviour change for existing users.

Shipped the same day v0.3.0 went LIVE on Chrome Web Store
(~15.5h after submission). Same-account update review window is
~24-48h per the reference_chrome_web_store_rules memory.

manifest.json

@@ -1,7 +1,7 @@
 {
   "manifest_version": 3,
   "name": "RangerHQ Tuner",
-  "version": "0.3.0",
+  "version": "0.4.0",
   "description": "Lightweight SomaFM radio player. Logs what plays. One-click search to Spotify, YouTube, Apple Music, Bandcamp. No telemetry.",
   "author": "David Keane",
   "homepage_url": "https://davidtkeane.com/rangerhq-tuner",