fix(modals): drop shadcn-style aria-describedby to silence Radix warning

Every DialogContent had aria-describedby="X-desc" and every
DialogDescription had a matching id="X-desc". That pattern is the
default shadcn/ui recipe, but it breaks Radix 1.1.x.

Root cause: Radix auto-generates a descriptionId via useId() (e.g. :r0:)
and stores it in the Dialog context. The DescriptionWarning effect does
`document.getElementById(descriptionId)` and warns if the element is
missing. When the description element's id is overridden to
"welcome-desc", the DOM has id="welcome-desc" but the context still has
:r0:, so the lookup misses and the warning fires on every dialog open.

Fix: drop both overrides. Let Radix manage the id and aria-describedby
itself. Screen-reader semantics are unchanged (Radix still wires
Content -> Description via the auto-generated id).

Touched: 12 modal files (11 DialogContent/DialogDescription pairs +
SettingsSheet which uses Sheet). +1 regression test in
WelcomeDialog.test.tsx that spies on console.warn and asserts the
"Missing Description" message is never emitted.

Tests: 306/306 pass (was 305).

Long-term memory: radix-aria-describedby-anti-pattern.md captures the
do-not-reintroduce rule for future agents.

tests/component/modals/WelcomeDialog.test.tsx

@@ -31,4 +31,27 @@ describe('WelcomeDialog', () => {
     await userEvent.click(screen.getByRole('button', { name: /get started/i }));
     expect(useSettingsStore.getState().welcomeDismissed).toBe(true);
   });
+
+  // Regression guard: Radix Dialog emits a console.warn when the Content's
+  // aria-describedby points at an id that isn't in the DOM. The default
+  // shadcn/ui pattern (custom id="X-desc" on DialogDescription) breaks that
+  // link because Radix's internal descriptionId is auto-generated via useId.
+  // The fix is to drop the override and let Radix manage the id itself.
+  it('does not emit the Radix "Missing Description" warning', async () => {
+    const warnings: string[] = [];
+    const spy = vi.spyOn(console, 'warn').mockImplementation((...args) => {
+      warnings.push(args.map(String).join(' '));
+    });
+    try {
+      render(<WelcomeDialog />);
+      // Effects that fire the warning run in a microtask; one tick is enough.
+      await Promise.resolve();
+      const offending = warnings.filter((w) =>
+        /Missing\s+`?Description`?|aria-describedby=\{undefined\}/.test(w),
+      );
+      expect(offending).toEqual([]);
+    } finally {
+      spy.mockRestore();
+    }
+  });
 });