release: rebuild .deb with cross-distro pkexec dependency

On Debian 13 (trixie), policykit-1 was retired and pkexec moved
into a standalone package. The previous Depends: policykit-1
made `apt install` fail with \"none of the choices are installable\"
on a fresh trixie VM.

Switch to Depends: pkexec | policykit-1 so apt can satisfy the
constraint on both:
  - bookworm: policykit-1 (which provides /usr/bin/pkexec)
  - trixie:   pkexec (standalone)

Also updates SHA256SUMS and the install README accordingly.

src-tauri/tauri.conf.json

@@ -40,7 +40,7 @@
     "externalBin": ["binaries/mycelium"],
     "linux": {
       "deb": {
-        "depends": ["policykit-1"],
+        "depends": ["pkexec | policykit-1"],
         "files": {
           "/usr/share/polkit-1/actions/tech.threefold.mycellium-ui.policy": "packaging/polkit/tech.threefold.mycellium-ui.policy"
         }