docs(store): add Chrome Web Store screenshots (4× 1280×800 PNG)

Web Store-spec screenshots for Dashboard upload, generated from
David's screen grabs taken on the live v0.3.0 build.

Screenshots (1280×800 PNG, the Chrome Web Store required size):
  1. Stations tab — DEF CON Radio + quick chips + browse list
  2. History tab — the headline 4-button-search feature with 4
     tracks already logged (Spotify / YouTube / Apple / Bandcamp)
  3. Favourites tab — starred-track persistence demo
  4. Toolbar popup — composed on a dark canvas, helmet icon
     visible in the Chrome address bar above

ImageMagick recipe:
  - NTP shots: crop bottom Chrome chrome (1378x950+0+0), resize
    to width 1280, north-anchored extent to 1280x800
  - Popup: resize to height 740, center on 1280x800 canvas
    with palette-matching #0f1411 background

Source originals archived under store/source/ for regeneration.

No code changes — extension behaviour unchanged.

store/screenshots/README.md

@@ -0,0 +1,36 @@
+# Chrome Web Store screenshots — RangerHQ Tuner v0.3.0
+
+Upload these in order at:
+**Dashboard → RangerHQ Tuner → Store listing → Screenshots**
+
+Spec met: **1280×800 PNG**, ≤ 5 images (Chrome Web Store requirement).
+
+| # | File | What it shows | Why it earns the slot |
+|---|---|---|---|
+| 1 | `screenshot-1-stations.png` | The full New Tab Page on the Stations tab — clock top-right (real-time), DEF CON Radio queued, Play button, eight quick-station chips, searchable browse list of SomaFM channels. | First impression. Shows the core product immediately. |
+| 2 | `screenshot-2-history.png` | The same New Tab Page on the **History** tab — 4 tracks logged with the **Spotify / YouTube / Apple / Bandcamp** search buttons per row clearly visible. | The headline feature — the single-purpose statement comes alive here. |
+| 3 | `screenshot-3-favourites.png` | The **Favourites** tab with a single starred track. | Demonstrates the star-toggle persistence and the per-user favourites surface. |
+| 4 | `screenshot-4-popup.png` | The **toolbar popup** in context (with the helmet icon visible in the Chrome address bar) showing the bottom nav row: **Open in tab / History / Settings**. | Anchors "where the extension lives" — the Chrome toolbar. |
+
+## Source files
+
+The original screen grabs (1376–1378 × 1155 PNGs) are archived at `../source/` and were processed with ImageMagick:
+
+```bash
+# NTP shots (1378×1156 → 1280×800)
+magick main-stations.png -crop 1378x950+0+0 -resize 1280x \
+  -gravity north -background "#0f1411" -extent 1280x800 screenshot-1-stations.png
+
+# Popup (367×590 → composed on 1280×800 dark canvas)
+magick extension.png -resize x740 \
+  -gravity center -background "#0f1411" -extent 1280x800 screenshot-4-popup.png
+```
+
+Same dark base colour `#0f1411` as the popup palette, so the Dashboard preview blends naturally with the extension's own visuals.
+
+## Optional follow-on assets (deferred)
+
+- **Promo tile** (440×280 PNG) — required by the Web Store for the listing tile in search/browse results. Not yet produced.
+- **Marquee tile** (1400×560 PNG) — optional but Google promotes extensions with marquee assets more often. Not yet produced.
+
+Both can be generated from `../source/` plus the helmet logo at `../../src/assets/img/ranger.png` whenever ready.