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/UsersPage.impl.tsx

@@ -891,9 +891,10 @@ export function UsersPage() {
 							<div className="flex flex-wrap gap-1">
 								<Tooltip>
 									<TooltipTrigger asChild>
-										<UiButton
+<UiButton
 											variant="ghost"
 											size="icon"
+											className="mobile-touch-target"
 											onClick={() => insertMarkup("<strong>", "</strong>")}
 											aria-label="Bold"
 										>
@@ -904,9 +905,10 @@ export function UsersPage() {
 								</Tooltip>
 								<Tooltip>
 									<TooltipTrigger asChild>
-										<UiButton
+<UiButton
 											variant="ghost"
 											size="icon"
+											className="mobile-touch-target"
 											onClick={() => insertMarkup("<em>", "</em>")}
 											aria-label="Italic"
 										>
@@ -917,9 +919,10 @@ export function UsersPage() {
 								</Tooltip>
 								<Tooltip>
 									<TooltipTrigger asChild>
-										<UiButton
+<UiButton
 											variant="ghost"
 											size="icon"
+											className="mobile-touch-target"
 											onClick={addLink}
 											aria-label="Link"
 										>
@@ -930,9 +933,10 @@ export function UsersPage() {
 								</Tooltip>
 								<Tooltip>
 									<TooltipTrigger asChild>
-										<UiButton
+<UiButton
 											variant="ghost"
 											size="icon"
+											className="mobile-touch-target"
 											onClick={() => insertMarkup("<ul><li>", "</li></ul>")}
 											aria-label="Bullet list"
 										>
@@ -994,7 +998,7 @@ export function UsersPage() {
 											type="button"
 											aria-label={`Remove ${file.name}`}
 											onClick={() => removeAttachment(index)}
-											className="inline-flex items-center text-current [&>svg]:size-3"
+											className="mobile-touch-target inline-flex items-center text-current [&>svg]:size-3"
 										>
 											<Trash2 />
 										</button>