feat(mysql): native SQL execution — fix #41 type decoding, in-process restore, .sql open-with (#42)

Fixes #41.

MySQL columns now decode by their declared type instead of a blind try_get chain,
so every non-string type (integers, DECIMAL, BIT, YEAR, DATE/TIME/DATETIME/TIMESTAMP)
renders its value instead of NULL. MySQL restore runs in-process over the sqlx driver
(no bundled MariaDB client, so MySQL 8 caching_sha2_password works) and the Clean toggle
is implemented. .sql files can be opened with Gridline from the OS file manager.

Verified: cargo test --lib 442 passed / 0 failed; bun run test 133 files / 1151 tests
exit 0; tsc clean; both live MySQL 8 integration tests pass with the bundled CLI hidden.

desktop/src-tauri/tauri.conf.json

@@ -27,6 +27,16 @@
     "active": true,
     "targets": "all",
     "resources": ["resources/pg_tools/*", "resources/mysql_tools/*"],
+    "fileAssociations": [
+      {
+        "ext": ["sql"],
+        "name": "SQL Script",
+        "description": "SQL script",
+        "mimeType": "application/sql",
+        "role": "Editor",
+        "rank": "Alternate"
+      }
+    ],
     "macOS": {
       "signingIdentity": "-",
       "hardenedRuntime": true,