chore(charts,errors): apply the tap-target floor across charts and error chrome

Final surface sweep for the v1.4.27 tap-target work: the chart band
overlay, the error-details action row, and the public-page header
links.

- `health-chart.tsx`: band overlay used to inset `right: 18px` while
  the underlying ComposedChart margin is `right: 8`, so the colour
  band drifted left of the plotted line by 10 px on every viewport.
  Pin the overlay to the chart's actual right edge.
- `error-details.tsx`: drop `size="sm"` from the retry, copy, and
  report-issue buttons so they sit at the default 40 px primitive
  height — clears the WCAG 2.5.5 floor on touch.
- `auth/login/page.tsx`: back-to-passkey toggle wraps in `inline-flex
  min-h-11 w-full` so the form-mode swap is reachable on touch.
- `privacy/page.tsx` and `about/page.tsx`: header HealthLog + Sign-in
  links wrap in `inline-flex min-h-11` so the public-page chrome
  clears the floor.

src/app/about/page.tsx

@@ -60,13 +60,13 @@ export default function AboutPage() {
         <div className="mx-auto flex max-w-3xl items-center justify-between px-4 py-3 md:px-6">
           <Link
             href="/"
-            className="text-foreground hover:text-primary text-sm font-semibold tracking-tight"
+            className="text-foreground hover:text-primary inline-flex min-h-11 items-center text-sm font-semibold tracking-tight"
           >
             HealthLog
           </Link>
           <Link
             href="/auth/login"
-            className="text-muted-foreground hover:text-foreground text-sm"
+            className="text-muted-foreground hover:text-foreground inline-flex min-h-11 items-center text-sm"
           >
             Sign in
           </Link>

src/app/auth/login/page.tsx

@@ -224,7 +224,7 @@ export default function LoginPage() {
               <button
                 type="button"
                 onClick={() => setMode("passkey")}
-                className="text-muted-foreground hover:text-foreground w-full text-center text-xs"
+                className="text-muted-foreground hover:text-foreground inline-flex min-h-11 w-full items-center justify-center text-center text-xs"
               >
                 {t("auth.backToPasskey")}
               </button>

src/app/privacy/page.tsx

@@ -111,13 +111,13 @@ export default function PrivacyPage() {
         <div className="mx-auto flex max-w-3xl items-center justify-between px-4 py-3 md:px-6">
           <Link
             href="/"
-            className="text-foreground hover:text-primary text-sm font-semibold tracking-tight"
+            className="text-foreground hover:text-primary inline-flex min-h-11 items-center text-sm font-semibold tracking-tight"
           >
             HealthLog
           </Link>
           <Link
             href="/auth/login"
-            className="text-muted-foreground hover:text-foreground text-sm"
+            className="text-muted-foreground hover:text-foreground inline-flex min-h-11 items-center text-sm"
           >
             Sign in
           </Link>