fix(ui): reserve Button loader space to eliminate CLS during in-flight requests

Lighthouse v1.4.33 run flagged a 0.186 CLS attached to a
`div.space-y-2 > Button > Loader-Circle` element. Root cause: the
two heavy submit buttons in `/bugreport` and the medications import
dialog drew `{loading && <Loader2/>}<Icon/>` -- rendering BOTH icons
during the in-flight request and growing the button by ~24 px. The
fix is to swap (not append) the leading icon, so Loader2 occupies
the same 16x16 slot the static icon would.

- src/app/bugreport/page.tsx: ternary swap on `loading`, pass
  `aria-busy` for screen-reader narration.
- src/app/medications/page.tsx (Import dialog): same shape.
- src/components/ui/button.tsx: document the loading-state contract
  on the primitive so future contributors apply the swap convention
  by default. (Callsites inside `src/components/settings/**` are
  owned by IW4 -- left untouched, the doc-comment makes the convention
  visible from the primitive.)

src/app/bugreport/page.tsx

@@ -227,9 +227,22 @@ export default function BugReportPage() {
           )}
 
           <div className="flex items-center justify-end pt-2">
-            <Button type="submit" disabled={loading}>
-              {loading && <Loader2 className="mr-2 h-4 w-4 animate-spin motion-reduce:animate-none" />}
-              <Bug className="mr-2 h-4 w-4" />
+            <Button type="submit" disabled={loading} aria-busy={loading || undefined}>
+              {/*
+                v1.4.33 IW9 — swap the icon on loading (Loader2 takes
+                the same 16x16 slot as the Bug icon) so the button
+                width is stable across the disabled flip. The previous
+                `{loading && <Loader2/>}<Bug/>` rendered both icons
+                during the in-flight request and grew the button by
+                ~24 px — the CLS regression flagged in the v1.4.33
+                Lighthouse punch list (0.186 on
+                `div.space-y-2 > Button > Loader-Circle`).
+              */}
+              {loading ? (
+                <Loader2 className="mr-2 h-4 w-4 animate-spin motion-reduce:animate-none" />
+              ) : (
+                <Bug className="mr-2 h-4 w-4" />
+              )}
               {t("bugreport.submit")}
             </Button>
           </div>

src/app/medications/page.tsx

@@ -505,9 +505,21 @@ function IntakeImportDialog({
               <Button
                 onClick={handleImport}
                 disabled={importing || !jsonText.trim()}
+                aria-busy={importing || undefined}
               >
-                {importing && <Loader2 className="mr-2 h-4 w-4 animate-spin motion-reduce:animate-none" />}
-                <Upload className="mr-2 h-4 w-4" />
+                {/*
+                  v1.4.33 IW9 — icon swap (not append) on `importing`
+                  to keep the button width stable through the
+                  in-flight request. Pre-v1.4.33 both icons painted
+                  during loading and the button grew by ~24 px, which
+                  showed up as a CLS hit on the medications import
+                  dialog.
+                */}
+                {importing ? (
+                  <Loader2 className="mr-2 h-4 w-4 animate-spin motion-reduce:animate-none" />
+                ) : (
+                  <Upload className="mr-2 h-4 w-4" />
+                )}
                 {t("common.import")}
               </Button>
             </div>

src/components/ui/button.tsx

@@ -4,6 +4,31 @@ import { Slot } from "radix-ui";
 
 import { cn } from "@/lib/utils";
 
+/**
+ * v1.4.33 IW9 — loading-state convention.
+ *
+ * Callers driving a transient loading state on a button must SWAP the
+ * leading icon for `<Loader2 />` rather than APPENDING the spinner
+ * next to the existing icon. The legacy `{loading && <Loader2/>}<Icon/>`
+ * pattern grew the button by ~24 px during the request, which produced
+ * the 0.186 CLS hit flagged on the v1.4.33 Lighthouse run. The
+ * spinner and the icon are both 16 px (`h-4 w-4`); swapping keeps the
+ * intrinsic button width stable.
+ *
+ *   ```tsx
+ *   <Button disabled={loading} aria-busy={loading || undefined}>
+ *     {loading ? (
+ *       <Loader2 className="mr-2 h-4 w-4 animate-spin motion-reduce:animate-none" />
+ *     ) : (
+ *       <Icon className="mr-2 h-4 w-4" />
+ *     )}
+ *     {label}
+ *   </Button>
+ *   ```
+ *
+ * Also: pass `aria-busy={loading || undefined}` so screen readers
+ * announce the in-flight state.
+ */
 const buttonVariants = cva(
   "inline-flex items-center justify-center gap-2 whitespace-nowrap rounded-md text-sm font-medium transition-all disabled:pointer-events-none disabled:opacity-50 [&_svg]:pointer-events-none [&_svg:not([class*='size-'])]:size-4 shrink-0 [&_svg]:shrink-0 outline-none focus-visible:border-ring focus-visible:ring-ring/50 focus-visible:ring-[3px] aria-invalid:ring-destructive/20 dark:aria-invalid:ring-destructive/40 aria-invalid:border-destructive",
   {