feat(branding): replace placeholder app icon with the real logo mark

Cropped the speech-bubble-with-sparkles mark from the new
WhispAssist logo/promo graphic (not the full graphic — text and
illustration detail don't read at 16-32px), color-keyed its background
to transparent, and regenerated the whole icon set via `tauri icon`
(title bar, taskbar, system tray, installer). Removed the iOS/Android/
Appx/macOS outputs tauri icon generates by default — this project only
targets Windows.

src-tauri/icons/README.md

@@ -1,4 +1,20 @@
 # Icons
 
-Placeholder icon set (Phase 1) generated with `npm run tauri icon`. Replace `icon.ico`/`tray.png`
-(and re-run `npm run tauri icon path/to/source.png`) with real branding before shipping.
+Real branding — a purple speech-bubble-with-sparkles mark cropped from the
+WhispAssist logo/promo graphic, background color-keyed to transparent and
+centered on a 512x512 canvas.
+
+To regenerate after a new source image: crop just the icon mark (not the
+full promo graphic — text and fine illustration detail don't read at 16-32px),
+then run:
+
+```
+npm run tauri icon path/to/mark.png
+```
+
+This only targets Windows (no iOS/Android/Appx bundle here), so delete
+`android/`, `ios/`, `icon.icns`, `Square*Logo.png`, and `StoreLogo.png` if
+`tauri icon` regenerates them — it defaults to generating icons for every
+platform regardless of what this project actually targets. `tray.png` is a
+manual 32x32 export (not one of `tauri icon`'s own output names) referenced
+directly by `lib.rs`'s system tray setup.