fork: initialize Mycellium UI Private from Mycell-UI@5229e2c

This repo is a hard fork of mycellium-ui dedicated to the
mycelium-private experimental track upstream. The two apps coexist
on the same machine via distinct app identifiers, polkit actions,
and binary names.

Renames
- package + crate: mycellium-ui → mycellium-ui-private
- bundle identifier: tech.threefold.mycellium-ui-private
- daemon binary: mycelium-private (separate upstream release tarball)
- bootstrap wrapper: /usr/bin/mycellium-bootstrap-private
- polkit policy file + action id

Functional changes
- SidecarConfig.network_name field (UTF-8, 2..=64 bytes)
- start() refuses to spawn without a network name AND a 32-byte
  key file at app_data_dir/network_key.bin; surfaces a clear
  error rather than letting mycelium-private fail mid-startup
- network_key_status / generate / import / export / delete
  commands; uses OS RNG (rand) and writes 0600
- empty default peers list (no Threefold seed for private overlays)
- new Settings → Private network panel: name input, key generate /
  reveal-hex / import / delete, status indicator

Adapted bootstrap script kills both `mycelium` and
`mycelium-private` orphans (cross-clash on UDP/9650 + TCP/8990).

CI workflow + sidebar branding updated. The README explains the
divergence model and how to cherry-pick upstream fixes.

src-tauri/packaging/polkit/tech.threefold.mycellium-ui-private.policy

@@ -0,0 +1,28 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE policyconfig PUBLIC
+ "-//freedesktop//DTD PolicyKit Policy Configuration 1.0//EN"
+ "https://www.freedesktop.org/standards/PolicyKit/1/policyconfig.dtd">
+<policyconfig>
+  <vendor>Threefold</vendor>
+  <vendor_url>https://threefold.io</vendor_url>
+
+  <!--
+    Bootstrap action: covers the wrapper that cleans up orphan
+    mycelium state and then execs the daemon. pkexec matches the
+    binary path against `org.freedesktop.policykit.exec.path` to
+    pick this action up; auth_admin_keep then caches the auth for
+    the user's session so subsequent restarts don't re-prompt.
+  -->
+  <action id="tech.threefold.mycellium-ui-private.bootstrap">
+    <description>Run the Mycelium overlay daemon</description>
+    <description xml:lang="fr">Lancer le démon de l'overlay Mycelium</description>
+    <message>Authentication is required to start the Mycelium overlay daemon.</message>
+    <message xml:lang="fr">Une authentification est requise pour démarrer le démon Mycelium.</message>
+    <defaults>
+      <allow_any>auth_admin</allow_any>
+      <allow_inactive>auth_admin</allow_inactive>
+      <allow_active>auth_admin_keep</allow_active>
+    </defaults>
+    <annotate key="org.freedesktop.policykit.exec.path">/usr/bin/mycellium-bootstrap-private</annotate>
+  </action>
+</policyconfig>