Fix SSH password authentication logic by removing requirePassword field

This commit eliminates the confusing requirePassword field that was causing
authentication issues where users couldn't disable password requirements.

Changes:
- Remove requirePassword field from database schema and migrations
- Simplify SSH authentication logic by removing special case branches
- Update frontend to remove requirePassword UI controls
- Clean up translation files to remove unused strings
- Support standard SSH empty password authentication

The new design follows the principle of "good taste" - password field itself
now expresses the requirement: null/empty = no password auth, value = use password.

Fixes the issue where setting requirePassword=false didn't work as expected.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/backend/database/db/index.ts

@@ -365,11 +365,6 @@ const migrateSchema = () => {
     "INTEGER REFERENCES ssh_credentials(id)",
   );
 
-  addColumnIfNotExists(
-    "ssh_data",
-    "require_password",
-    "INTEGER NOT NULL DEFAULT 1",
-  );
 
   // SSH credentials table migrations for encryption support
   addColumnIfNotExists("ssh_credentials", "private_key", "TEXT");