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.

frontend/package-lock.json

@@ -1,11 +1,11 @@
 {
-  "name": "nost-feed-manager",
+  "name": "keynectr",
   "version": "0.1.0",
   "lockfileVersion": 3,
   "requires": true,
   "packages": {
     "": {
-      "name": "nost-feed-manager",
+      "name": "keynectr",
       "version": "0.1.0",
       "dependencies": {
         "react": "^18.3.1",