Add system tray, APK Installer, fix package visibility, new themes/fonts; bump to v1.3.0

- System tray (Linux): pure Go D-Bus StatusNotifierItem, no cgo/libappindicator.
  Close now offers Minimize to Tray vs Quit instead of always quitting.
- New APK Installer module: batch-install from a folder or hand-picked files,
  live per-file progress, select all/none.
- Fix Packages/App Inspector silently hiding packages disabled/uninstalled by
  Canta/Shizuku (pm list packages needs -u to see them). Added a state filter
  and per-row Restore/Enable/Disable/Uninstall actions.
- 6 new theme palettes (Nord, Tokyo Night, Solarized Dark/Light, Rosé Pine,
  Everforest - 26 total), custom text colour, 8 bundled display fonts,
  adjustable font size.
- Horizontal sidebar now shrinks label text progressively instead of
  scrolling when the window narrows; fixed a touchpad stuck-scrollbar-drag bug.
- Updated README and fixed stale Go 1.23 -> 1.25 build instructions.

tray_windows.go

@@ -0,0 +1,10 @@
+package main
+
+// No tray implementation on Windows yet - only verified on Linux (see
+// tray_linux.go) against this project's actual dev/test machine. ATK runs
+// fine without it; the frontend's close dialog just won't offer "Minimize to
+// tray" here (TrayAvailable() is false).
+
+func initTray(a *App)   {}
+func closeTray()        {}
+func trayAvailable() bool { return false }