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.json

@@ -1,5 +1,6 @@
 {
-  "name": "nost-feed-manager",
+  "name": "keynectr",
+  "productName": "Keynectr",
   "productName": "Nostr Feed Manager",
   "version": "0.1.0",
   "description": "A friendly Linux desktop app for managing Nostr profiles and publishing text notes.",
@@ -58,8 +59,8 @@
     ],
     "extraResources": [
       {
-        "from": "../target/release/nostr-manager-backend",
-        "to": "nostr-manager-backend"
+        "from": "../target/release/keynectr",
+        "to": "keynectr"
       }
     ],
     "linux": {
@@ -67,7 +68,7 @@
         "dir"
       ],
       "category": "Network",
-      "executableName": "nost-feed-manager"
+      "executableName": "keynectr"
     }
   }
 }