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/src/integration.rs

@@ -85,6 +85,11 @@ pub fn integrate_desktop() -> Result<IntegrationStatus, String> {
 
     // .desktop launcher -> user applications dir. Icon/Exec are absolute paths, so no
     // icon-theme lookup is needed.
+    //
+    // StartupWMClass is the exception: it must be the BINARY name, because GTK
+    // derives the window's WM_CLASS from the executable and the desktop matches the
+    // two to group the taskbar icon. Not the product name, and not this AppImage's
+    // filename — AppRun execs `usr/bin/thoughtsync` inside it (issue 2075).
     let entry = desktop_entry_path().ok_or("Cannot resolve the applications dir.")?;
     if let Some(parent) = entry.parent() {
         fs::create_dir_all(parent).map_err(|e| format!("create applications dir: {e}"))?;
@@ -98,7 +103,7 @@ pub fn integrate_desktop() -> Result<IntegrationStatus, String> {
          Icon={icon}\n\
          Terminal=false\n\
          Categories=Utility;Office;\n\
-         StartupWMClass=ThoughtSync\n",
+         StartupWMClass=thoughtsync\n",
         exec = installed.to_string_lossy(),
         icon = icon.to_string_lossy(),
     );