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/utils/database-sqlite-export.ts

@@ -126,7 +126,6 @@ class DatabaseSQLiteExport {
               pin INTEGER NOT NULL DEFAULT 0,
               auth_type TEXT NOT NULL,
               password TEXT,
-              require_password INTEGER NOT NULL DEFAULT 1,
               key TEXT,
               key_password TEXT,
               key_type TEXT,
@@ -225,7 +224,6 @@ class DatabaseSQLiteExport {
               const fieldMappings: Record<string, string> = {
                 userId: "user_id",
                 authType: "auth_type",
-                requirePassword: "require_password",
                 keyPassword: "key_password",
                 keyType: "key_type",
                 credentialId: "credential_id",
@@ -464,7 +462,6 @@ class DatabaseSQLiteExport {
               const columnToFieldMappings: Record<string, string> = {
                 user_id: "userId",
                 auth_type: "authType",
-                require_password: "requirePassword",
                 key_password: "keyPassword",
                 key_type: "keyType",
                 credential_id: "credentialId",