v0.6.0: release workflow, constraint-aware cell editing, pending-cell styling (#9)

* feat: demo DB seed/regenerate, backup/restore/sync refinements, SSH/SSL polish

- Demo SQLite DB: feature-rich seed (12 objects, 500-row audit log) + regenerate action in Settings
- Backup/restore: headless-testable core logic, psql -f for plain dumps, sync passes --clean --if-exists
- SSH/SSL runtime refinements and connection testing improvements
- Data grid: DataTypeIcon component, FK popover, table tree polish
- Docs: AGENTS.md/README updates, new screenshots, MIT LICENSE

* chore: optimize README screenshots (4.7 MB → 916 KB via pngquant, quality 70-90)

* v0.6.0: release workflow, constraint-aware cell editing, pending-cell styling

- Bump version to 0.6.0 across package.json, Cargo.toml, tauri.conf.json
- Add .github/workflows/release.yml: tag-triggered CI builds macOS (aarch64
  + x64), Windows, and Linux installers into a draft GitHub Release
- README: installer download table (unsigned note, per-platform files),
  "how releases are made" section
- CellEditor: constraint-aware commit — empty input on nullable columns
  becomes NULL, NOT NULL text-like types fall back to empty string, all
  other types blocked with an inline error bubble; replace "Set NULL"
  checkbox with a NULL row in the FK dropdown / empty enum option
- VirtualDataGrid: pending-edit dot → animated pending outline (ring) on
  staged cells; matching test updates
- docs-coverage test: align with rewritten README comparison table

src-tauri/src/commands/ssh.rs

@@ -165,13 +165,16 @@ impl TunnelBackend for Ssh2Backend {
             .map_err(|e| format!("connect ssh host: {e}"))?;
         let mut session = Session::new().map_err(|e| format!("ssh session: {e}"))?;
         session.set_tcp_stream(tcp);
-        session.handshake().map_err(|e| format!("ssh handshake: {e}"))?;
+        session
+            .handshake()
+            .map_err(|e| format!("ssh handshake: {e}"))?;
 
         match cfg.auth_method.as_str() {
             "key" => {
-                let path = cfg.private_key_path.as_deref().ok_or_else(|| {
-                    "private_key_path required for key auth".to_string()
-                })?;
+                let path = cfg
+                    .private_key_path
+                    .as_deref()
+                    .ok_or_else(|| "private_key_path required for key auth".to_string())?;
                 session
                     .userauth_pubkey_file(&cfg.user, None, std::path::Path::new(path), passphrase)
                     .map_err(|e| format!("ssh key auth: {e}"))?;
@@ -420,4 +423,4 @@ mod tests {
         mgr.close_all();
         assert_eq!(mgr.active_count(), 0);
     }
-}
+}