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.

src/renderer/components/modals/ExportDocxDialog.tsx

@@ -41,10 +41,10 @@ export function ExportDocxDialog({ sourcePath }: { sourcePath: string }) {
 
   return (
     <Dialog open onOpenChange={(o) => !o && closeModal()}>
-      <DialogContent aria-describedby="docx-desc">
+      <DialogContent>
         <DialogHeader>
           <DialogTitle>Export to DOCX</DialogTitle>
-          <DialogDescription id="docx-desc">{sourcePath}</DialogDescription>
+          <DialogDescription>{sourcePath}</DialogDescription>
         </DialogHeader>
         <div className="space-y-3 text-sm">
           <div>