P5: per-platform transparent, size-appropriate window/tray icons

Give the window titlebar, taskbar, and tray a size-appropriate icon with a
transparent boundary on both Windows and Linux, instead of scaling one PNG to
every size.

Assets:
- Regenerate the PNGs and gosentry.ico with feathered color-to-alpha so the
  rounded-tile boundary is transparent; a binary white-key had left an opaque
  halo that read as a border on dark taskbars/trays.
- Rebuild gosentry.ico as multi-size (16 hand-tuned + 32/48/256 from big) and
  add a single-frame 16x16 gosentry-icon-16x16.ico for the Windows tray.
- assets.go: add IconSmall() and IconSmallICO().

Wiring:
- Windows window/taskbar: embed gosentry.ico under the GLFW_ICON resource and
  skip a.SetIcon so GLFW selects the right frame per size (hand-tuned 16 for the
  titlebar, a larger frame for the taskbar).
- Windows tray: SetSystemTrayIcon(IconSmallICO()), a 16x16 ICO frame.
- Linux window titlebar: a.SetIcon(IconSmall()) for a crisp ~16px _NET_WM_ICON;
  tray uses the big PNG since StatusNotifierItem renders larger.

Also bump Fyne 2.6.3 -> 2.7.4 (systray 1.11.0 -> 1.12.1) and document the full
cross-platform icon strategy in assets.go, gosentry.rc, run.go, and tray.go.

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

packaging/windows/gosentry.rc

@@ -1 +1,10 @@
+// The same multi-size .ico is embedded under two resource names on purpose:
+//   IDI_ICON1 - the lowest-id icon, which Windows Explorer / shortcuts use for the
+//               executable's file icon.
+//   GLFW_ICON - the name GLFW looks up to icon the window. Because run.go does NOT
+//               call a.SetIcon on Windows, GLFW falls back to this resource and
+//               selects the right frame per size (hand-tuned 16 for the titlebar,
+//               a larger frame for the taskbar). See assets/assets.go for the full
+//               cross-platform icon strategy.
 IDI_ICON1 ICON "assets/gosentry.ico"
+GLFW_ICON ICON "assets/gosentry.ico"