desktop: one name across all three install channels (issue 2075)

The app answered to three different names depending on how it arrived, and the
part that actually hurt was WM_CLASS. Reading tauri-bundler settles what it is:
the generated .desktop template writes StartupWMClass={{exec}} where exec is
main_binary_name, and tao creates its GtkApplication with a NULL app id
(enableGTKAppId defaults off), so GTK falls back to the program name. WM_CLASS
is the binary name, nothing else.

Which inverts this issue's premise. The rename could not break grouping,
because two channels weren't grouping in the first place: pacman ships
/usr/bin/thoughtsync and the AppImage's AppRun execs thoughtsync-desktop, while
all three hand-written entries hardcoded StartupWMClass=ThoughtSync — a string
no binary in any channel has ever reported. Only the .deb worked, and only
because Tauri generates its entry from the binary and never consulted us.

So: thoughtsync everywhere, carried by the build target itself via Cargo [[bin]]
plus mainBinaryName rather than by the install path, since the target name is
what the desktop reads. The pacman package sheds its -desktop suffix and
declares conflict+replaces so an upgrade retires the old one instead of landing
beside it and fighting over /usr/bin/thoughtsync.

The .deb verifier now asserts binary path, Exec and StartupWMClass all agree,
which is the part that keeps this fixed: the .deb's entry is the one no human
writes, so it's the one that drifts silently.

Package: thought-sync stays. tauri-bundler derives it as kebab-case(productName)
with no override, and rewriting a control archive on every build is a poor trade
for one uninstall command.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

desktop/src-tauri/tauri.conf.json

@@ -1,6 +1,7 @@
 {
   "$schema": "https://schema.tauri.app/config/2",
   "productName": "ThoughtSync",
+  "mainBinaryName": "thoughtsync",
   "version": "0.1.0",
   "identifier": "com.fabledsword.thoughtsync",
   "build": {