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_other.go

@@ -0,0 +1,6 @@
+//go:build !windows
+
+package ui
+
+// screenSize is unknown off Windows; the caller falls back to the spec size.
+func screenSize() (int, int) { return 0, 0 }