Display arrangement persistence + centering fix, packaging hardening

- core/display.py: write_positions() persists Arrangement's drag-and-drop
  layout into niri's config.kdl (one output { position x= y= } per output),
  validated via `niri validate` on a temp copy with a .kdl.bak backup before
  writing — same pattern as core/keybindings.py's rebind(). Previously the
  page only ever called `niri msg output … position set`, which niri treats
  as live-only and drops on the next login/reload.
- gui/pages/display.py: Arrangement's Apply now runs each output's `niri msg
  output … position set` synchronously instead of queuing them all on the
  single-shot ProcessRunner (which rejects a second run() while the first is
  still async) — a 2-monitor apply previously moved only the first output
  and silently dropped the rest. _dock_to_nearest keeps the free axis at the
  dragged position (so a shorter display can sit vertically centered next to
  a taller rotated one) rather than forcing corner alignment.
- core/panel.py, files/__init__.py: incidental fixes alongside the above.
- packaging/: signing-key generation script + build-user systemd setup for
  the [tanin] AUR auto-rebuild pipeline; PKGBUILD bumped to pkgrel=5.
- src/taninux/browser/: new module for browser theme sync (Fuji accent).

README.md

@@ -25,7 +25,10 @@ The central `taninux` app is organized around five pillars:
 | `tsettings` | Settings app (network, display, audio, users, …) |
 | `thub` | Software Hub (repo/AUR/flatpak install) |
 | `tfiles` | Native GTK4 file manager |
-| `tmusic` | Music app |
+
+`tmusic` (TIDAL client) has moved to its own repo, `taninux-music`
+(GPL-3.0, depends on `taninux[gui]`), and is no longer part of the base
+ISO.
 
 The GTK apps run on a custom libadwaita-free backend
 (`src/taninux/gui/ui/native.py` + `style_apple.css`); `TANINUX_UI=adw`