docs(repo): add directory readmes

apps/README.md

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+# Apps
+
+Runnable DBX applications live here.
+
+## Directories
+
+- `desktop/` - the Vue frontend used by the Tauri desktop app and the Docker/web build.
+
+The Tauri native shell remains in `src-tauri/` because that is the conventional Tauri project location used by the existing build and release tooling.

crates/README.md

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+# Crates
+
+Rust crates for DBX live here.
+
+## Directories
+
+- `dbx-core/` - shared database core, drivers, schema/query logic, import/export, transfer, and plugin support.
+- `dbx-web/` - the Docker/web backend service binary published as `dbx-web`.
+
+The workspace root is defined in the repository-level `Cargo.toml`.

plugins/README.md

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+# Plugins
+
+DBX extension plugins live here.
+
+Plugins are not Node packages, so they intentionally stay separate from `packages/`.
+
+## Directories
+
+- `jdbc/` - optional Java/JDBC plugin for connecting through vendor JDBC drivers and custom JDBC URLs.
+
+Plugin build artifacts, downloaded driver JARs, and local targets are ignored by Git.