M10 (task 2022, issue 2021): Arch pacman package + commit icon set (native, system libs)

Native-first fix for the AppImage black window: a PKGBUILD that builds from source,
linked against SYSTEM graphics libs, so it renders on the host driver.
- desktop/packaging/arch/{PKGBUILD,thoughtsync.desktop,README.md}: `makepkg -si`
  installs /usr/bin/thoughtsync + .desktop + icon; deps webkit2gtk-4.1/gtk3/...;
  builds the frontend + `cargo build --release` (no tauri-cli). Uses the host
  graphics stack -> avoids EGL_BAD_PARAMETER.
- Commit the icon set (desktop/src-tauri/icons/*.png, un-gitignored) so BOTH
  `cargo build` (pacman) and `cargo tauri build` (deb/appimage) work without a
  generate step; bundle.icon -> the 4 committed PNGs; drop the `cargo tauri icon`
  step from desktop.yml.
- Broaden the WebKit software-render hardening (lib.rs) to ALL Linux (was
  AppImage-scoped) so the native build also renders if system WebKit is finicky.

Can't CI-test the PKGBUILD (Arch-only; CI is Debian) -- operator builds locally.
desktop.yml re-verifies the deb+AppImage build with the committed icons.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FRgehjoz7Yv8LkUfADxACm

desktop/src-tauri/.gitignore

@@ -3,7 +3,3 @@
 
 # Generated by Tauri
 /gen/schemas
-
-# App icons are generated from ../../frontend/public/icon.svg via `cargo tauri
-# icon` (in CI before the build); not committed.
-/icons/