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/icons/hicolor/scalable/apps/io.github.outlay.svg

@@ -1,21 +0,0 @@
-<?xml version="1.0" encoding="UTF-8"?>
-<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 128 128" width="128" height="128">
-  <defs>
-    <linearGradient id="bg" x1="0" y1="0" x2="0" y2="1">
-      <stop offset="0%" stop-color="#3584e4"/>
-      <stop offset="100%" stop-color="#1a73e8"/>
-    </linearGradient>
-  </defs>
-  <!-- Background circle -->
-  <circle cx="64" cy="64" r="60" fill="url(#bg)"/>
-  <!-- Wallet body -->
-  <rect x="24" y="38" width="72" height="52" rx="8" fill="#ffffff" opacity="0.95"/>
-  <!-- Wallet flap -->
-  <path d="M24 50 Q24 38 36 38 L88 38 Q96 38 96 46 L96 50 Z" fill="#e8e8e8"/>
-  <!-- Coin circle -->
-  <circle cx="82" cy="64" r="14" fill="#f5c211" stroke="#e0a800" stroke-width="2"/>
-  <!-- Dollar sign on coin -->
-  <text x="82" y="70" font-family="sans-serif" font-size="18" font-weight="bold" fill="#8b6914" text-anchor="middle">$</text>
-  <!-- Arrow out (expense indicator) -->
-  <path d="M40 64 L54 54 L54 58 L66 58 L66 70 L54 70 L54 74 Z" fill="#2ec27e" opacity="0.9"/>
-</svg>