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/packaging/arch/package-prebuilt.sh

@@ -29,10 +29,14 @@ SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
 REPO_ROOT="$(cd "$SCRIPT_DIR/../../.." && pwd)"
 
 SRC_TAURI="$REPO_ROOT/desktop/src-tauri"
-BINARY="$SRC_TAURI/target/release/thoughtsync-desktop"
+BINARY="$SRC_TAURI/target/release/thoughtsync"
 OUT_DIR="${1:-$SRC_TAURI/target/release/bundle/arch}"
 
-PKGNAME="thoughtsync-desktop"
+PKGNAME="thoughtsync"
+# The name this package used to ship under. pacman needs both to retire it: without
+# them a `pacman -U` of the renamed package installs ALONGSIDE the old one, and two
+# packages both own /usr/bin/thoughtsync (issue 2075).
+REPLACES=(thoughtsync-desktop)
 PKGREL=1
 PKGDESC="ThoughtSync desktop — local-first Keep-style thought capture"
 URL="https://git.fabledsword.com/bvandeusen/thoughtsync"
@@ -73,9 +77,10 @@ STAGE="$(mktemp -d)"
 trap 'rm -rf "$STAGE"' EXIT INT TERM
 
 # --- lay out the filesystem tree --------------------------------------------
-# /usr/bin/thoughtsync (not thoughtsync-desktop): matches the CLI name the
-# AppImage installer symlinks into ~/.local/bin, so the command is the same
-# whichever way the app was installed.
+# /usr/bin/thoughtsync — the same command name the .deb installs and the AppImage
+# installer symlinks into ~/.local/bin, so it's identical whichever way the app
+# arrived. The binary already carries this name (Cargo `[[bin]]`), which is also
+# what the .desktop entry's StartupWMClass has to match.
 install -Dm755 "$BINARY" "$STAGE/usr/bin/thoughtsync"
 install -Dm644 "$SCRIPT_DIR/thoughtsync.desktop" \
   "$STAGE/usr/share/applications/thoughtsync.desktop"
@@ -109,6 +114,10 @@ INSTALLED_SIZE="$(du -sb "$STAGE" | cut -f1)"
   echo "arch = x86_64"
   echo "license = $LICENSE"
   for d in "${DEPENDS[@]}"; do echo "depend = $d"; done
+  # conflict + replaces together: `conflict` is what makes pacman remove the old
+  # package rather than refuse the transaction, `replaces` is what makes an upgrade
+  # pick this one up under its new name.
+  for r in "${REPLACES[@]}"; do echo "conflict = $r"; echo "replaces = $r"; done
 } >"$STAGE/.PKGINFO"
 
 # --- .MTREE (optional) ------------------------------------------------------