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/settings.rs

@@ -18,7 +18,11 @@ pub fn get_settings(state: tauri::State<crate::AppState>) -> Result<Settings, St
 }
 
 #[tauri::command]
-pub fn update_setting(state: tauri::State<crate::AppState>, key: String, value: String) -> Result<(), String> {
+pub fn update_setting(
+    state: tauri::State<crate::AppState>,
+    key: String,
+    value: String,
+) -> Result<(), String> {
     update_setting_inner(&state.db_store, &key, &value)
 }
 
@@ -55,4 +59,4 @@ mod tests {
         update_setting_inner(&st, "accent_color", "#EF4444").unwrap();
         assert_eq!(get_settings_inner(&st).unwrap().accent_color, "#EF4444");
     }
-}
+}