Change app ID to com.outlay.app, add AppStream metadata, fix toast visibility

- Change app ID from io.github.outlay to com.outlay.app across all files
- Add AppStream metainfo with full feature description, 16 screenshots, and v0.1.0 release
- Update desktop file with expanded metadata (StartupNotify, SingleMainWindow)
- Add summary and description fields to GSchema keys
- Move toast overlay outside ScrolledWindow so notifications stay visible in viewport
- Embed tray icon as ARGB pixmap data for reliable system tray display
- Register hicolor icon theme path for taskbar icon on Wayland
- Remove unused icon variants (old naming, web favicons, SVG, ICO, shadow)
- Add screenshots to data/screenshots/
- Update build script with metainfo and screenshot bundling

outlay-gtk/data/com.outlay.app.desktop

@@ -0,0 +1,11 @@
+[Desktop Entry]
+Name=Outlay
+Comment=Personal income and expense tracker with budgets, charts, and multi-currency support
+Exec=outlay-gtk
+Icon=com.outlay.app
+Terminal=false
+Type=Application
+Categories=Office;Finance;GTK;
+Keywords=expense;budget;money;finance;income;tracker;currency;receipt;
+StartupNotify=true
+SingleMainWindow=true