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/src/commands/open_files.rs

@@ -0,0 +1,121 @@
+//! OS-delivered "open this .sql file with Gridline" requests.
+//!
+//! Every platform funnels into [`accept_path`], which validates strictly and
+//! queues the path. The frontend only ever calls [`take_pending_sql_files`],
+//! which takes **no path argument** and drains what was queued — so the webview
+//! cannot name a file to read. That is the boundary that keeps this feature
+//! from becoming an arbitrary-filesystem-read primitive and defeating the
+//! deliberate `fs` scope (spec §2).
+
+use std::path::{Path, PathBuf};
+use std::sync::Mutex;
+use tauri::State;
+
+/// Editor-load cap. A larger script is routed to Tools → Restore instead of
+/// being handed to Monaco (spec §2).
+pub const MAX_SQL_FILE_BYTES: u64 = 5 * 1024 * 1024;
+
+/// A validated open request, delivered to the frontend with its contents.
+#[derive(Debug, Clone, serde::Serialize)]
+#[serde(rename_all = "camelCase")]
+pub struct PendingSqlFile {
+    pub path: String,
+    pub name: String,
+    pub content: String,
+}
+
+/// Process-wide FIFO of accepted paths. Lives in Rust (not the webview) so a
+/// window reload cannot lose an open request.
+#[derive(Default)]
+pub struct PendingOpenFiles(Mutex<Vec<PathBuf>>);
+
+/// Strict validation. Fail closed: anything we are not certain about is
+/// rejected rather than opened (spec §6).
+pub fn validate_sql_path_with_cap(raw: &str, cap: u64) -> Option<PathBuf> {
+    let path = Path::new(raw);
+
+    // Absolute only — the OS always hands us absolute paths, and accepting a
+    // relative one would resolve against the process CWD. Note the app is
+    // often launched with CWD `/` (see the app-data-dir note in lib.rs).
+    if !path.is_absolute() {
+        return None;
+    }
+
+    let ext = path.extension()?.to_str()?;
+    if !ext.eq_ignore_ascii_case("sql") {
+        return None;
+    }
+
+    // symlink_metadata, not metadata: a symlink must not be followed into an
+    // arbitrary target (and `file_type().is_file()` is false for one).
+    let meta = std::fs::symlink_metadata(path).ok()?;
+    if !meta.file_type().is_file() {
+        return None;
+    }
+    if meta.len() > cap {
+        return None;
+    }
+    Some(path.to_path_buf())
+}
+
+/// Validate against the production cap.
+pub fn validate_sql_path(raw: &str) -> Option<PathBuf> {
+    validate_sql_path_with_cap(raw, MAX_SQL_FILE_BYTES)
+}
+
+/// Queue an OS-supplied path. Returns `true` when it was newly queued, so
+/// callers can decide whether a nudge event is worth emitting.
+pub fn accept_path(buffer: &PendingOpenFiles, raw: &str) -> bool {
+    let Some(path) = validate_sql_path(raw) else {
+        return false;
+    };
+    let mut queue = buffer.0.lock().unwrap();
+    if queue.iter().any(|p| p == &path) {
+        return false;
+    }
+    queue.push(path);
+    true
+}
+
+/// Read and remove everything queued. Separated from the command so it is
+/// testable without a Tauri `State`.
+pub fn take_from_buffer(buffer: &PendingOpenFiles) -> Result<Vec<PendingSqlFile>, String> {
+    let drained: Vec<PathBuf> = {
+        let mut queue = buffer.0.lock().unwrap();
+        std::mem::take(&mut *queue)
+    };
+
+    drained
+        .into_iter()
+        .map(|path| {
+            let name = path
+                .file_name()
+                .map(|n| n.to_string_lossy().into_owned())
+                .unwrap_or_else(|| path.to_string_lossy().into_owned());
+            // Fail closed on undecodable bytes rather than guessing an
+            // encoding for the user's SQL.
+            let bytes =
+                std::fs::read(&path).map_err(|e| format!("Cannot read {}: {e}", name))?;
+            let content =
+                String::from_utf8(bytes).map_err(|_| format!("{name} is not valid UTF-8"))?;
+            Ok(PendingSqlFile {
+                path: path.to_string_lossy().into_owned(),
+                name,
+                content,
+            })
+        })
+        .collect()
+}
+
+/// Drain every queued open request. The only file-reading command the webview
+/// can call — deliberately argument-free (see the module docs).
+#[tauri::command]
+pub fn take_pending_sql_files(
+    state: State<'_, PendingOpenFiles>,
+) -> Result<Vec<PendingSqlFile>, String> {
+    take_from_buffer(&state)
+}
+
+#[cfg(test)]
+#[path = "open_files.test.rs"]
+mod open_files_tests;