Fix: Keep ConfigPage tabs mounted to preserve form state

Previously, the tab content wrapper used 'key={tab}' which caused React to
unmount and remount the entire subtree when switching tabs. This destroyed
all component state, including unsaved form data and pending auto-saves.

Changes:
- Removed 'key={tab}' from the wrapper div
- All tab panels now render at page initialization
- Inactive tabs use CSS 'display: none' to hide without unmounting
- Tabs remain mounted throughout the page lifetime
- Users can now switch tabs without losing form input

Updated ConfigPage.test.tsx to reflect that inactive tabs remain in the DOM
(just hidden with CSS) rather than being removed entirely.

Documentation: Added 'Tab Panels' section to Web-Development.md
explaining the rule and rationale.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

frontend/src/pages/__tests__/ConfigPage.test.tsx

@@ -38,7 +38,8 @@ describe("ConfigPage", () => {
     renderPage();
     fireEvent.click(screen.getByRole("tab", { name: /filters/i }));
     expect(screen.getByTestId("filters-tab")).toBeInTheDocument();
-    expect(screen.queryByTestId("jails-tab")).not.toBeInTheDocument();
+    // Jails tab remains mounted (not removed from DOM), just hidden with CSS
+    expect(screen.getByTestId("jails-tab")).toBeInTheDocument();
   });
 
   it("switches to Actions tab when Actions tab is clicked", () => {