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)

apps/desktop/src/components/connection/ConnectionDialog.vue

@@ -142,6 +142,7 @@ const defaultForm = (): ConnectionForm => ({
   redis_sentinel_tls: false,
   redis_cluster_nodes: "",
   etcd_endpoints: "",
+  read_only: false,
 });
 
 function defaultSshTunnel(): SshTunnelConfig {
@@ -573,6 +574,7 @@ watch(
         redis_sentinel_tls: config.redis_sentinel_tls || false,
         redis_cluster_nodes: config.redis_cluster_nodes || "",
         etcd_endpoints: config.etcd_endpoints || "",
+        read_only: config.read_only || false,
       };
       h2ConnectionMode.value = h2ConnectionModeForConfig(config);
       customColorInput.value = config.color || "";
@@ -1069,6 +1071,7 @@ function connectionConfigForSubmit(id: string): ConnectionConfig {
   const idleTimeout = Number(config.idle_timeout_secs);
   config.idle_timeout_secs = Number.isFinite(idleTimeout) && idleTimeout >= 0 ? idleTimeout : 60;
   if (!config.one_time) config.one_time = undefined;
+  if (!config.read_only) config.read_only = undefined;
   if (config.db_type === "mongodb" && !mongoUseUrl.value) {
     config.connection_string = undefined;
   } else if (config.db_type === "mongodb") {
@@ -3130,6 +3133,13 @@ function openExternalUrl(url: string) {
                     class="col-span-3"
                   />
                 </div>
+                <div class="grid grid-cols-4 items-center gap-4">
+                  <Label class="text-right text-xs">{{ t("connection.readOnly") }}</Label>
+                  <label class="col-span-3 flex items-center gap-2 cursor-pointer">
+                    <input type="checkbox" v-model="form.read_only" class="mr-0" />
+                    <span class="text-xs text-muted-foreground">{{ t("connection.readOnlyHint") }}</span>
+                  </label>
+                </div>
               </div>
             </TabsContent>