refactor: convert to bun-workspaces monorepo (desktop/ + www/)

- Move the Tauri app (React frontend + Rust backend) into desktop/ via
  git mv — configs unchanged (relative paths: tauri.conf.json, vite.config.ts)
- Root package.json becomes a private bun workspace container with
  orchestration scripts; desktop package renamed gridline-desktop
- Scaffold www/ with Astro 7 + Tailwind v4 (hand-wired vite plugin,
  static output, ready for Dokploy)
- Update release.yml for the new layout: projectPath: desktop,
  desktop/src-tauri resource paths, rust-cache workspace path
- Update README + AGENTS.md structure trees and dev commands
- Verified: vitest (1074), cargo test (354), desktop build, tauri dev
  (window launches), tauri build (dmg + app bundles), Astro build

desktop/src-tauri/resources/mysql_tools/README.md

@@ -0,0 +1,13 @@
+# Bundled MariaDB client tools
+
+Static `mariadb-dump` and `mariadb` client binaries (wire-compatible with MySQL
+servers) go here. They are NOT committed — `.github/workflows/release.yml`
+builds and checksum-verifies them per matrix target, copying the result into
+`resources/mysql_tools/{mariadb-dump,mariadb}` (plus `mariadb-dump.exe` /
+`mariadb.exe` and the runtime `libmariadb.dll` on Windows). The app resolves
+tools system-first and falls back to these bundled binaries (see
+`backup::resolve_mysql_tool`).
+
+In `tauri dev`, this dir is usually empty — the app falls back to any
+`mariadb-dump`/`mariadb` on `PATH` (system-first resolution). MySQL
+backup/restore degrades with a clear error if neither is present.

desktop/src-tauri/resources/pg_tools/README.md

@@ -0,0 +1,8 @@
+# Bundled PostgreSQL client tools
+
+This directory is populated **at build time by CI** (`.github/workflows/release.yml`)
+with statically-linked `pg_dump`, `pg_restore`, and `psql` for the current target.
+Binaries are NOT committed to the repo.
+
+In `tauri dev`, this dir is usually empty — the app falls back to any `pg_dump`/`pg_restore`
+on `PATH` (system-first resolution). Admin features degrade with a clear error if neither is present.