feat(v2): T-308A window 80% of screen, 2x3 gallery, Esc close, button states

- Window opens at 80% of the primary display (Windows GetSystemMetrics).
- Gallery is now 2 columns x 3 rows and the right panel is narrower.
- Esc closes the full-image viewer (key focus + filter).
- Start/Stop grey out per running state (gtx.Disabled drops input); a hard
  ReportIssue resets running so Start re-enables. Cross-compiles for Windows.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

v2/internal/ui/screen_windows.go

@@ -0,0 +1,18 @@
+//go:build windows
+
+package ui
+
+// SM_CXSCREEN / SM_CYSCREEN return the primary display size in pixels.
+const (
+	smCXScreen = 0
+	smCYScreen = 1
+)
+
+var procGetSystemMetrics = user32.NewProc("GetSystemMetrics")
+
+// screenSize returns the primary display size in pixels, or (0, 0) if unavailable.
+func screenSize() (int, int) {
+	width, _, _ := procGetSystemMetrics.Call(uintptr(smCXScreen))
+	height, _, _ := procGetSystemMetrics.Call(uintptr(smCYScreen))
+	return int(width), int(height)
+}