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/ConfigPage.tsx

@@ -36,7 +36,11 @@ const useStyles = makeStyles({
   header: {
     marginBottom: tokens.spacingVerticalL,
   },
-  tabContent: {
+  tabPanel: {
+    display: "none",
+  },
+  tabPanelVisible: {
+    display: "block",
     marginTop: tokens.spacingVerticalL,
     animationName: "fadeInUp",
     animationDuration: tokens.durationNormal,
@@ -94,16 +98,26 @@ export function ConfigPage(): React.JSX.Element {
         <Tab value="regex">Regex Tester</Tab>
       </TabList>
 
-      <div className={styles.tabContent} key={tab}>
-        {tab === "jails" && (
-          <JailsTab
-            initialJail={(location.state as { jail?: string } | null)?.jail}
-          />
-        )}
-        {tab === "filters" && <FiltersTab />}
-        {tab === "actions" && <ActionsTab />}
-        {tab === "server" && <ServerTab />}
-        {tab === "regex" && <RegexTesterTab />}
+      <div className={tab === "jails" ? styles.tabPanelVisible : styles.tabPanel}>
+        <JailsTab
+          initialJail={(location.state as { jail?: string } | null)?.jail}
+        />
+      </div>
+
+      <div className={tab === "filters" ? styles.tabPanelVisible : styles.tabPanel}>
+        <FiltersTab />
+      </div>
+
+      <div className={tab === "actions" ? styles.tabPanelVisible : styles.tabPanel}>
+        <ActionsTab />
+      </div>
+
+      <div className={tab === "server" ? styles.tabPanelVisible : styles.tabPanel}>
+        <ServerTab />
+      </div>
+
+      <div className={tab === "regex" ? styles.tabPanelVisible : styles.tabPanel}>
+        <RegexTesterTab />
       </div>
     </div>
   );

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", () => {