fix(desktop): align H5 mobile composer and diff scrolling

H5 exposes the desktop chat surfaces in a much narrower viewport, so the empty-session composer needs the same inset treatment as active sessions and generated diff rows need intrinsic width instead of compressing long code into the phone viewport.

Constraint: Keep desktop layout and short diff-row backgrounds unchanged while enabling sideways scrolling on H5.
Rejected: Force wrapping long diff lines | it makes code harder to inspect and diverges from desktop diff behavior.
Confidence: high
Scope-risk: narrow
Directive: Keep workspace diff rows content-width driven when adjusting mobile code preview styling.
Tested: cd desktop && bun run test -- src/pages/EmptySession.test.tsx --run
Tested: cd desktop && bun run test -- src/components/workspace/WorkspacePanel.test.tsx --run
Tested: cd desktop && bun run lint
Not-tested: Full bun run verify gate; change is scoped to desktop UI styling and covered by narrow Vitest plus typecheck.

desktop/src/components/workspace/WorkspaceCodeSurface.tsx

@@ -127,7 +127,7 @@ export function WorkspaceDiffSurface({
             return (
               <div
                 key={index}
-                className={`grid grid-cols-[48px_18px_minmax(0,1fr)] gap-2 px-3 ${
+                className={`grid min-w-full w-max grid-cols-[48px_18px_max-content] gap-2 px-3 ${
                   isAdded
                     ? 'bg-[var(--color-diff-added-bg)]'
                     : isRemoved

desktop/src/components/workspace/WorkspacePanel.test.tsx

@@ -763,6 +763,57 @@ describe('WorkspacePanel', () => {
     expect(view.getByRole('button', { name: 'Collapse preview' })).toBeTruthy()
   })
 
+  it('keeps diff rows intrinsically wide so H5 users can scroll sideways', async () => {
+    const longDiffLine = '+const label = "this is a very long generated line that should not be compressed into the phone viewport";'
+
+    await setWorkspaceState((state) => ({
+      ...state,
+      panelBySession: {
+        ...state.panelBySession,
+        'session-wide-diff': {
+          isOpen: true,
+          activeView: 'changed',
+          hasUserSelectedView: true,
+        },
+      },
+      statusBySession: {
+        ...state.statusBySession,
+        'session-wide-diff': {
+          state: 'ok',
+          workDir: '/repo',
+          repoName: 'repo',
+          branch: 'main',
+          isGitRepo: true,
+          changedFiles: [],
+        },
+      },
+      previewTabsBySession: {
+        ...state.previewTabsBySession,
+        'session-wide-diff': [{
+          id: 'diff:wide.ts',
+          path: 'wide.ts',
+          kind: 'diff',
+          title: 'wide.ts',
+          diff: longDiffLine,
+          state: 'ok',
+        }],
+      },
+      activePreviewTabIdBySession: {
+        ...state.activePreviewTabIdBySession,
+        'session-wide-diff': 'diff:wide.ts',
+      },
+    }))
+
+    const view = await renderPanel('session-wide-diff')
+    const diffSurface = view.getByTestId('workspace-code')
+    const firstRow = diffSurface.querySelector('div')
+
+    expect(firstRow?.className).toContain('w-max')
+    expect(firstRow?.className).toContain('min-w-full')
+    expect(firstRow?.className).toContain('grid-cols-[48px_18px_max-content]')
+    expect(diffSurface.textContent).toContain(longDiffLine)
+  })
+
   it('can expand long file previews beyond the default rendered line cap', async () => {
     const longFile = Array.from({ length: 2300 }, (_, index) => `const line${index + 1} = ${index + 1}`).join('\n')
 

desktop/src/pages/EmptySession.test.tsx

@@ -194,6 +194,8 @@ describe('EmptySession', () => {
     })
     expect(screen.getByTestId('model-selector')).toHaveAttribute('data-compact', 'true')
     expect(screen.getByRole('button', { name: 'Run' })).toHaveClass('h-11', 'w-11')
+    expect(screen.getByTestId('empty-session-composer-shell')).toHaveClass('px-3')
+    expect(screen.getByTestId('empty-session-composer-panel')).toHaveClass('rounded-2xl')
   })
 
   it('creates a session with the selected project and branch when submitted', async () => {

desktop/src/pages/EmptySession.tsx

@@ -541,15 +541,19 @@ export function EmptySession() {
         </div>
       </div>
 
-      <div className={`absolute left-0 right-0 z-30 flex justify-center ${
+      <div
+        data-testid="empty-session-composer-shell"
+        className={`absolute left-0 right-0 z-30 flex justify-center ${
         isMobileComposer
-          ? 'bottom-0 px-0 pb-0'
+          ? 'bottom-0 px-3 pb-[calc(env(safe-area-inset-bottom)+10px)]'
           : 'bottom-4 px-8'
-      }`}>
+      }`}
+      >
         <div className={`flex w-full flex-col ${isMobileComposer ? 'max-w-none' : 'max-w-3xl'}`}>
           <div
+            data-testid="empty-session-composer-panel"
             className={`glass-panel relative flex flex-col gap-3 ${
-              isMobileComposer ? 'rounded-t-2xl rounded-b-none p-3 shadow-[0_-12px_36px_rgba(54,35,28,0.12)]' : 'rounded-t-xl rounded-b-none p-4'
+              isMobileComposer ? 'rounded-2xl p-3 shadow-[0_-12px_36px_rgba(54,35,28,0.12)]' : 'rounded-t-xl rounded-b-none p-4'
             }`}
             onDragOver={(event) => event.preventDefault()}
             onDrop={handleDrop}