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>

src/ui/tray.go

@@ -1,6 +1,10 @@
 package ui
 
 import (
+	"runtime"
+
+	"gitea.mixdep.ru/mix/gosentry/assets"
+
 	"fyne.io/fyne/v2"
 	fynedesktop "fyne.io/fyne/v2/driver/desktop"
 )
@@ -13,6 +17,17 @@ func configureSystemTray(a fyne.App, w fyne.Window) {
 		return
 	}
 
+	// The tray icon is platform-specific. The Windows notification area is
+	// ICO-native and renders at 16-24px, so a single 16x16 .ico frame keeps the
+	// hand-tuned glyph crisp with correct alpha. Linux/macOS trays
+	// (StatusNotifierItem) render much larger (22-48px) and take a PNG, so the
+	// full-size artwork scales down cleanly instead of upscaling the tiny 16x16.
+	if runtime.GOOS == "windows" {
+		desk.SetSystemTrayIcon(assets.IconSmallICO())
+	} else {
+		desk.SetSystemTrayIcon(assets.Icon())
+	}
+
 	// IsQuit marks this as the tray's quit item. Without it Fyne's
 	// addMissingQuitForMenu appends a second, localized Quit (e.g. "Выход" on a
 	// Russian system) because it only recognizes an existing quit by matching the