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/Cargo.lock

@@ -1946,11 +1946,13 @@ dependencies = [
 ]
 
 [[package]]
-name = "mycellium-ui"
+name = "mycellium-ui-private"
 version = "0.1.0"
 dependencies = [
+ "hex",
  "parking_lot",
  "portpicker",
+ "rand 0.8.6",
  "reqwest 0.12.28",
  "serde",
  "serde_json",