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.gschema.xml

@@ -0,0 +1,20 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<schemalist gettext-domain="outlay">
+  <schema id="com.outlay.app" path="/com/outlay/app/">
+    <key name="window-width" type="i">
+      <default>900</default>
+      <summary>Window width</summary>
+      <description>The width of the main application window.</description>
+    </key>
+    <key name="window-height" type="i">
+      <default>600</default>
+      <summary>Window height</summary>
+      <description>The height of the main application window.</description>
+    </key>
+    <key name="window-maximized" type="b">
+      <default>false</default>
+      <summary>Window maximized</summary>
+      <description>Whether the main application window is maximized.</description>
+    </key>
+  </schema>
+</schemalist>