Rename the app to Keynectr

- Crate/binary: nostr-manager-backend -> keynectr
- Data directory: nost-feed-manager -> keynectr, migrated automatically
  on first data_dir() call (existing vaults, settings and backups move)
- Electron extraResources/spawn path, executableName, productName,
  window title and CLI usage strings updated to match
- Deliberately unchanged: crypto.rs KDF verifier string, so previously
  encrypted vault backups remain decryptable

Verified live: existing vault with two profiles migrated to
~/.local/share/keynectr and loads correctly.

Cargo.lock

@@ -683,6 +683,25 @@ dependencies = [
  "wasm-bindgen",
 ]
 
+[[package]]
+name = "keynectr"
+version = "0.1.0"
+dependencies = [
+ "aes-gcm",
+ "argon2",
+ "base64",
+ "getrandom 0.2.17",
+ "hex",
+ "nostr",
+ "nostr-sdk",
+ "rpassword",
+ "serde",
+ "serde_json",
+ "tokio",
+ "uuid",
+ "zeroize",
+]
+
 [[package]]
 name = "libc"
 version = "0.2.189"
@@ -781,25 +800,6 @@ dependencies = [
  "tokio",
 ]
 
-[[package]]
-name = "nostr-manager-backend"
-version = "0.1.0"
-dependencies = [
- "aes-gcm",
- "argon2",
- "base64",
- "getrandom 0.2.17",
- "hex",
- "nostr",
- "nostr-sdk",
- "rpassword",
- "serde",
- "serde_json",
- "tokio",
- "uuid",
- "zeroize",
-]
-
 [[package]]
 name = "nostr-relay-pool"
 version = "0.40.1"