fix(bundle): normalize Exec/TryExec in the embedded desktop entries

[FeiLiuEM]

Follow-up to #168 — the DBus-related parts of my first attempt (#169, closed) were wrong, and Samueru-sama's reply was right: quick-sharun already strips DBusActivatable for the entry that matters, and every integration tool rewrites Exec/TryExec when it installs the entry (AM, GearLever, Soar, AppImageLauncher). I am not claiming any user-visible breakage here.

What this PR does (and nothing else): normalize the absolute build path that the entries shipped inside the image carry.

Upstream dist/linux/app.desktop.in uses the @GHOSTTY@ placeholder, and zig build substitutes it with the absolute build prefix. bundle-appimage.sh copies ghostty-<ver>/zig-out/share/* into AppDir/share/ (quick-sharun copies the same file to the AppDir root), so both entries end up with:

TryExec=/__w/ghostty-appimage/ghostty-appimage/ghostty-<ver>/zig-out/bin/ghostty
Exec=/__w/ghostty-appimage/ghostty-appimage/ghostty-<ver>/zig-out/bin/ghostty --gtk-single-instance=true

The change rewrites those two keys to the binary name the README documents (install ./Ghostty-*.AppImage $HOME/.local/bin/ghostty), so the entry that ships inside the image is portable on its own. This is also what the repo's own assets/ghostty.desktop did before 98e5a30 ("chore: restructure project") switched to consuming upstream's generated file.

Deliberately not touched (the mistakes of #169):

• DBusActivatable — quick-sharun handles the AppDir root entry.
• share/dbus-1/services/com.mitchellh.ghostty.service — left in place.

Verification

Ran the new block against the files extracted from the current tip AppImage (Ghostty-1.3.2-main+a4aacd9-x86_64, sha256 0ff324551791220ec4a5d41cb562e7f2b272559cfb2c4ff0288a81c911939a57):

before → AppDir/com.mitchellh.ghostty.desktop:            TryExec=/__w/…, Exec=/__w/… --gtk-single-instance=true
after  →                                                  TryExec=ghostty, Exec=ghostty --gtk-single-instance=true
before → AppDir/share/applications/com.mitchellh.ghostty.desktop: same paths (+ DBusActivatable=true)
after  →                                                  same rewrite, DBusActivatable left as shipped
[Desktop Action new-window] Exec= is rewritten as well (the sed applies per line)
desktop-file-validate: clean on both entries
shellcheck --severity=warning --shell=sh bin/bundle-appimage.sh: clean
shfmt -d -s bin/bundle-appimage.sh: no diff

The full CI build container was not run locally; the patch only edits files that already exist in AppDir at that point.

If you'd rather not carry this at all, say so and I'll close it — as established in #168, nothing depends on it.