Touch-target audit: 44px minimum on mobile interactive elements (Slice 9)

Apply the mobile-touch-target CSS class to 40 interactive elements across
12 files. The class applies min-height/min-width:44px only below md
(max-width:767px), satisfying WCAG 2.5.5 / Apple HIG on touch devices.
Desktop behavior is unchanged.

Audit log (before -> after hit-area):
- App.tsx: hamburger/dark-mode/sign-out (32/32/28 -> 44)
- Dashboard.tsx: shortcut open/edit/delete (28 -> 44), enabled switch (18 -> 44)
- Media.tsx: mobile pagination prev/next (28 -> 44)
- FileBrowser.impl.tsx: 'Open Settings' alert button (28 -> 44)
- UsersPage.impl.tsx: compose toolbar bold/italic/link/list (32 -> 44),
  attachment remove button (16 -> 44)
- Settings.tsx: machine switch (18 -> 44), clear/add-machine buttons (28 -> 44),
  reset-db checkboxes x3 (16 -> 44)
- Actions.tsx: 'Add action' button (28 -> 44)
- ServicePage.tsx: service enabled switch (18 -> 44)
- ServicesPage.tsx: service switch/open-link/delete-icon (18/28/32 -> 44)
- ObservabilityPage.tsx: retry + 4 asChild link buttons (28 -> 44)
- WidgetConfigDialog.tsx: 4 icon buttons (32 -> 44), 2 switches (18 -> 44),
  2 add-widget buttons (28 -> 44)
- SessionActivityPanel.tsx: 'Open in Users' button (28 -> 44)

Deliberately skipped: default-size text buttons (32px, borderline), desktop-only
sidebar toggle, DataTable internals (desktop-only below md), Select triggers.
Dashboard anchor pills and HoverEditButton already had the class from Slices 1/2.

No new tests (the class applies via @media which jsdom doesn't honor).
116 tests pass; lint/build green.

Refs openspec/changes/mobile-responsive-parity/ (spec R6, tasks slice 9).

frontend/src/components/ObservabilityPage.tsx

@@ -141,7 +141,7 @@ function QueryError({
 			<AlertTitle>{label} failed</AlertTitle>
 			<AlertDescription className="flex flex-col gap-3 sm:flex-row sm:items-center sm:justify-between">
 				<span className="break-words">{error.message}</span>
-				<Button variant="outline" size="sm" onClick={() => refetch()}>
+				<Button variant="outline" size="sm" className="mobile-touch-target" onClick={() => refetch()}>
 					<RefreshCw className="mr-1 h-3 w-3" />
 					Retry
 				</Button>
@@ -255,7 +255,7 @@ function GrafanaLinkCard({
 					<div className="font-medium">{title}</div>
 					<div className="text-sm text-muted-foreground">{description}</div>
 				</div>
-				<Button variant="outline" size="sm" asChild>
+				<Button variant="outline" size="sm" className="mobile-touch-target" asChild>
 					<a
 						href={href}
 						target="_blank"
@@ -585,7 +585,7 @@ export function ObservabilityPage() {
 									title="No Node Exporter targets"
 									description="Enable Node Exporter on an SSH machine in Settings to populate Prometheus scrape targets."
 									action={
-										<Button variant="outline" size="sm" asChild>
+										<Button variant="outline" size="sm" className="mobile-touch-target" asChild>
 											<Link to="/settings">Open Settings</Link>
 										</Button>
 									}
@@ -641,7 +641,7 @@ export function ObservabilityPage() {
 									title="No Grafana service configured"
 									description="Add a Grafana service instance to enable deep-links to dashboards and logs."
 									action={
-										<Button variant="outline" size="sm" asChild>
+										<Button variant="outline" size="sm" className="mobile-touch-target" asChild>
 											<Link to="/services">Open Services</Link>
 										</Button>
 									}
@@ -653,7 +653,7 @@ export function ObservabilityPage() {
 								title="No machine selected"
 								description="Add monitoring machines in Settings to see Grafana drill-down links."
 								action={
-									<Button variant="outline" size="sm" asChild>
+									<Button variant="outline" size="sm" className="mobile-touch-target" asChild>
 										<Link to="/settings">Open Settings</Link>
 									</Button>
 								}