feat(packaging): pre-spawn cleanup wrapper for clean restarts

Symptom: each app restart that didn't go through Stop daemon left
an orphan mycelium running as root, claiming the TUN \"mycelium\",
UDP/9650 (multicast discovery) and TCP/8990 (JSON-RPC, hardcoded
in 0.6.1 — no flag). Subsequent starts panicked with EBUSY or
\"Address in use\" on whichever port the orphan held.

We can't SIGKILL the orphan from user-space (root process). Move
the cleanup into an elevated context that runs in the same pkexec
authentication as the daemon spawn:

  /usr/bin/mycellium-bootstrap   (new shell script in the .deb)
    pkill -9 -x mycelium
    ip link del mycelium / mycel0
    exec /usr/bin/mycelium \"\$@\"

The polkit policy now annotates this exact path with
auth_admin_keep so a single password prompt covers every
subsequent restart in the user's session.

Sidecar: when /usr/bin/mycellium-bootstrap exists (production
install) we hand pkexec that path instead of the bare daemon.
\`pnpm tauri dev\` falls back to the unwrapped binary path.

src-tauri/tauri.conf.json

@@ -42,7 +42,8 @@
       "deb": {
         "depends": ["pkexec | policykit-1"],
         "files": {
-          "/usr/share/polkit-1/actions/tech.threefold.mycellium-ui.policy": "packaging/polkit/tech.threefold.mycellium-ui.policy"
+          "/usr/share/polkit-1/actions/tech.threefold.mycellium-ui.policy": "packaging/polkit/tech.threefold.mycellium-ui.policy",
+          "/usr/bin/mycellium-bootstrap": "packaging/mycellium-bootstrap"
         }
       }
     }