feat: add tray layout controls

BUILD.md

@@ -18,6 +18,8 @@ The project uses pure Go and sets `CGO_ENABLED=0`. No C compiler, Windows SDK,
 
 ```text
 cmd/fancywin/                          CLI entry point
+assets/fancywin.png                    Full-resolution generated icon source
+internal/assets/                       Embedded runtime image assets
 internal/config/                       YAML model, loading, and validation
 internal/layout/                       Platform-neutral zone geometry
 internal/platform/platform_windows.go  Win32 backend
@@ -56,8 +58,10 @@ The script:
 5. Writes `dist\fancywin.exe` and copies the example to
    `dist\fancywin.yaml`.
 
-The generated executable is a console subsystem application so users can see
-configuration and Win32 errors directly.
+The generated executable remains a console-subsystem application so diagnostics
+work without a second binary. Normal execution calls `FreeConsole` and operates
+through the notification-area icon. `--debug` retains the console, while
+startup failures in detached mode are shown with `MessageBoxW`.
 
 ## Cross-compiling
 
@@ -138,6 +142,13 @@ Test both x64 and ARM64 where hardware is available:
     same-integrity elevated run can do so.
 11. Disconnect and reconnect a monitor and confirm current work-area information
     is used for subsequent moves.
+12. Confirm the custom icon is crisp in the normal and overflow notification
+    areas at 100%, 150%, and 200% scaling.
+13. Select each named layout from the tray, confirm the check mark and overlay
+    change immediately, and verify `active_layout` changes without disturbing
+    YAML comments or layout formatting.
+14. Right-click the tray icon, select `Exit`, and confirm the process and icon
+    both disappear cleanly.
 
 ## Architecture
 
@@ -153,13 +164,29 @@ At startup the backend:
 
 1. Requests per-monitor-v2 DPI awareness.
 2. Creates a named mutex for single-instance enforcement.
-3. Installs a `SetWinEventHook` covering `EVENT_SYSTEM_MOVESIZESTART` through
+3. Creates a hidden window and registers a notification-area icon with
+   `Shell_NotifyIconW`.
+4. Installs a `SetWinEventHook` covering `EVENT_SYSTEM_MOVESIZESTART` through
    `EVENT_SYSTEM_MOVESIZEEND` with `WINEVENT_OUTOFCONTEXT` and
    `WINEVENT_SKIPOWNPROCESS`.
-4. Registers configured system-wide hotkeys against the current thread.
-5. Creates a two-second timer for configuration reload checks and a 25 ms timer
+5. Registers configured system-wide hotkeys against the current thread.
+6. Creates a two-second timer for configuration reload checks and a 25 ms timer
    for sampling the mouse button, Shift, and active drag state.
-6. Enters a standard Win32 `GetMessage` loop.
+7. Enters a standard Win32 `GetMessage` loop.
+
+The tray icon is a generated FancyWin mark embedded as a 64×64 RGBA PNG. At
+startup the standard-library PNG decoder supplies premultiplied BGRA pixels to a
+32-bit GDI DIB section, which `CreateIconIndirect` converts to an alpha-aware
+native `HICON`; it never needs to be extracted to disk. A stock Windows icon is
+used only if conversion unexpectedly fails.
+
+The tooltip contains the active layout. The context menu deduplicates configured
+layouts by case-insensitive name, checks the active one, and provides `Exit`.
+Selecting a layout updates runtime state immediately and rewrites only the
+top-level `active_layout` YAML line so the choice persists. `Exit` posts
+`WM_QUIT` to the existing message loop for orderly hook, hotkey, overlay, icon,
+and tray cleanup. A registered `TaskbarCreated` message restores the icon after
+Windows Explorer restarts.
 
 ### Mouse snapping