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/src/test/SettingsScreen.test.tsx

@@ -11,7 +11,7 @@ describe('SettingsScreen', () => {
     renderWithApp(<SettingsScreen />);
 
     expect(
-      await screen.findByText('/home/user/.local/share/nost-feed-manager/profiles_vault.json'),
+      await screen.findByText('/home/user/.local/share/keynectr/profiles_vault.json'),
     ).toBeInTheDocument();
     expect(screen.getByText('Storage is not encrypted')).toBeInTheDocument();
   });

frontend/src/test/apiMock.ts

@@ -35,8 +35,8 @@ export function makeState(overrides?: Partial<AppState>): AppState {
   };
   return {
     version: '0.1.0',
-    vault_path: '/home/user/.local/share/nost-feed-manager/profiles_vault.json',
-    settings_path: '/home/user/.local/share/nost-feed-manager/settings.json',
+    vault_path: '/home/user/.local/share/keynectr/profiles_vault.json',
+    settings_path: '/home/user/.local/share/keynectr/settings.json',
     encrypted_storage: false,
     vault_locked: false,
     migrated_from: null,
@@ -188,7 +188,7 @@ export function createApiMock(initial: AppState = makeState()): ApiMock {
       },
     ),
     backupNow: vi.fn(async () => ({
-      backup_path: '/home/user/.local/share/nost-feed-manager/profiles_vault.json.backup-1',
+      backup_path: '/home/user/.local/share/keynectr/profiles_vault.json.backup-1',
     })),
     setVaultPassword: vi.fn(async () => ({
       ...state,