fix(tools): resolve external MySQL/PG client tools correctly (#44)

Addresses #44 (closed after the v0.8.3 release builds are green).

Three independent defects in one shared tool-resolution path: bundled tools were
looked up at the wrong path (so bundled fallback never worked, for PostgreSQL
too), macOS GUI apps inherit launchd's minimal PATH, and only mariadb-dump/mariadb
were probed — never mysqldump/mysql, which is what brew install mysql-client
installs. Both engines now resolve through one headless, cached, timed resolver,
plus a non-blocking client/server version warning.

Also fixes passing MySQL-only CLI options to the bundled MariaDB client, and
carries the tool-page UI polish and the requested #40 icon revert.

ROADMAP.md

@@ -134,7 +134,7 @@ Still deferred from 0.7.0, slated for this bucket:
 ## ✅ Shipped (0.7.8)
 
 - **SQLite `.dump` support** — backup a SQLite database to a portable SQL dump and restore it back, matching the pg_dump UX
-- **Backup / Restore / Sync for MySQL & SQLite** — the pg-only tooling extended: MySQL backup/restore via `mysqldump` (system-first) and DB-to-DB sync beyond PostgreSQL
+- **Backup / Restore / Sync for MySQL & SQLite** — the pg-only tooling extended: MySQL backup via the resolved `mariadb-dump`/`mysqldump` client (system-first, bundled fallback) with a native in-process restore, plus DB-to-DB sync beyond PostgreSQL
 - **Excel (.xlsx) export** — alongside CSV/JSON/SQL/Markdown in the grid export toolbar
 - **Cancel long-running queries** — per-connection cancel button (`pg_cancel_backend` and equivalents) instead of waiting or killing the app
 - **Settings export / import** — share theme, accent, editor options, page sizes, and defaults across machines (JSON file)