feat(readonly): add read-only mode for all database connections

## Background
Add a read-only toggle for database connections that blocks all write
operations (INSERT, UPDATE, DELETE, DROP, etc.) when enabled, allowing
only read queries (SELECT, SHOW, EXPLAIN, etc.). Closes #889.

## Changes

### Data Model
- Added `read_only: bool` field to ConnectionConfig (Rust struct, TS
  interfaces, and ConnectionConfigData deserialization layer)
- Used `#[serde(default, skip_serializing_if = "is_false")]` for
  backward/forward compatibility with existing configs

### Frontend UI
- Added "Read Only" checkbox in connection dialog
- 7-language i18n support (en/es/it/pt-BR/zh-CN/zh-TW)

### SQL Classification (query_execution_sql.rs)
- Added `is_write_sql()` with two-layer defense:
  - Layer 1: First-keyword check (must start with known read keyword)
  - Layer 2: Embedded dangerous keyword detection (catches CTE-wrapped
    writes like `WITH ... AS (DELETE FROM ...)`)
  - `FROM` keyword supported as DuckDB SELECT-less syntax indicator
- Added `check_read_only()` returning descriptive error with connection name
- Added 16 unit tests covering: pure reads/writes, CTE, case insensitivity,
  string literal masking, comment stripping, stored procedure calls, edge cases

### SQL Execution Guards (query.rs / transfer.rs)
- Added `check_read_only_for_connection()` and `_multi()` helper functions
  with lazy name clone (only allocates when read_only is true)
- 6 interception points: `do_execute`, MySQL batch, SQL Server batch,
  DuckDB batch, transaction execution, transfer execution

### Non-SQL Write Guards — Tauri Commands
- Added `ensure_connection_writable()` helper (connection.rs)
- Mongo: 6 write entry points (insert/update/delete, single + batch)
- Redis: 15 write entry points (SET, DEL, HSET, HDEL, LPUSH, LSET, LREM,
  SADD, SREM, ZADD, ZREM, EXPIRE, FLUSHDB, delete_keys, execute_command)
  - execute_command uses RedisCommandSafety classification to allow
    safe read commands through raw command interface
- etcd: 2 write entry points (put, delete)
- sql_file: SQL file execution guarded
- MCP Bridge: 4 write entry points (Insert/Update/Delete/SQL query)

### Non-SQL Write Guards — Web API
- Local `ensure_writable()` helper in each route module
- Redis: 12 write endpoints including classified execute_command
- Mongo: 6 write endpoints
- etcd: 2 write endpoints
- sql_file, table_import, transfer: early rejection

### Test Updates
- Updated ConnectionConfig construction in 7 test files with
  `read_only: false` initialization

## Defense-in-Depth
- Layer 1: Command/Route-level early rejection (saves resources)
- Layer 2: Core SQL classifier (`is_write_sql` — first keyword + embedded scan)
- Layer 3: Core execution-time interception (do_execute/transfer/transaction)

src-tauri/src/commands/transfer.rs

@@ -1,7 +1,7 @@
 use std::sync::Arc;
 use tauri::{AppHandle, Emitter, State};
 
-use crate::commands::connection::AppState;
+use crate::commands::connection::{ensure_connection_writable, AppState};
 
 // Re-export types and functions used by other modules
 pub use dbx_core::transfer::{get_db_type, TransferProgress, TransferRequest, TransferStatus};
@@ -19,6 +19,9 @@ pub async fn start_transfer(
     let state = state.inner().clone();
     let transfer_id = request.transfer_id.clone();
 
+    // Reject transfer early if the target connection is read-only — writing to it is inherently required
+    ensure_connection_writable(&state, &request.target_connection_id, "Transfer").await?;
+
     // Validate connections exist
     let source_db_type = get_db_type(&state, &request.source_connection_id).await?;
     let target_db_type = get_db_type(&state, &request.target_connection_id).await?;