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/components/db-viewer/DbViewerScreen.test.tsx

@@ -148,12 +148,12 @@ describe("DbViewerScreen", () => {
         const staged = useDbViewerStore.getState().changesQueue[0];
         expect(staged?.table).toBe("users");
         expect(staged?.schema).toBe("public");
-        // grid cell shows the optimistic value + the pending dot (2nd match is the queue panel diff)
+        // grid cell shows the optimistic value + the pending outline (2nd match is the queue panel diff)
         await waitFor(() => {
             expect(screen.getAllByText("Alicia").length).toBeGreaterThanOrEqual(2);
         });
-        expect(screen.getByTestId("pending-edit-dot")).toBeInTheDocument();
-        // committing the change clears the pending dot but keeps the value until refetch
+        expect(screen.getByTestId("pending-cell")).toBeInTheDocument();
+        // committing the change clears the pending outline but keeps the value until refetch
         act(() => {
             useDbViewerStore
                 .getState()
@@ -162,7 +162,7 @@ describe("DbViewerScreen", () => {
                 );
         });
         await waitFor(() => {
-            expect(screen.queryByTestId("pending-edit-dot")).toBeNull();
+            expect(screen.queryByTestId("pending-cell")).toBeNull();
         });
         expect(screen.getAllByText("Alicia").length).toBeGreaterThanOrEqual(2);
         // clearing the queue clears the optimistic display