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/pages/Settings.tsx

@@ -232,6 +232,7 @@ function MachineEditor({
 						<div className="flex items-center gap-2">
 							<Switch
 								id="machine-enabled"
+								className="mobile-touch-target"
 								checked={draft.enabled}
 								onCheckedChange={(checked) =>
 									setDraft((current) => ({ ...current, enabled: checked }))
@@ -534,7 +535,7 @@ function SSHKeyManager({
 					<Button
 						variant="outline"
 						size="sm"
-						className="w-full"
+						className="mobile-touch-target w-full"
 						onClick={() => {
 							clear();
 						}}
@@ -782,6 +783,7 @@ function ResetLocalDatabaseCard() {
 					<div className="flex flex-col gap-3">
 						<label className="flex items-center gap-2 text-sm">
 							<Checkbox
+								className="mobile-touch-target"
 								checked={ackSettings}
 								onCheckedChange={(checked) => setAckSettings(Boolean(checked))}
 							/>
@@ -789,6 +791,7 @@ function ResetLocalDatabaseCard() {
 						</label>
 						<label className="flex items-center gap-2 text-sm">
 							<Checkbox
+								className="mobile-touch-target"
 								checked={ackIndex}
 								onCheckedChange={(checked) => setAckIndex(Boolean(checked))}
 							/>
@@ -796,6 +799,7 @@ function ResetLocalDatabaseCard() {
 						</label>
 						<label className="flex items-center gap-2 text-sm">
 							<Checkbox
+								className="mobile-touch-target"
 								checked={ackIrreversible}
 								onCheckedChange={(checked) =>
 									setAckIrreversible(Boolean(checked))
@@ -973,7 +977,7 @@ export function Settings() {
 										<Button
 											variant="outline"
 											size="sm"
-											className="w-full"
+											className="mobile-touch-target w-full"
 											onClick={() => {
 												clearSSHValidation();
 												setMachineDraft(emptyMachine("local"));