feat(settings): add Stacks section for stack workflow preferences (#1366)

Move the browser-local Deploy progress, Progress style, and Diff preview
before save controls out of Appearance into a new Stacks section under the
Infrastructure group. These are stack lifecycle and editor workflow
preferences, not visual style, so Settings now groups them where operators
expect to find them.

Add a browser-local masthead scope so these localStorage-backed sections read
SCOPE browser instead of the misleading global, and apply it to both
Appearance and Stacks. Control behavior, storage keys, and backing hooks are
unchanged; this is an information-architecture move only.

docs/features/appearance.mdx

@@ -50,5 +50,5 @@ The **Display** group holds the remaining per-browser preferences:
 
 - **Density** switches between **Comfortable** (roomy rows, the default) and **Compact** (tighter rows and tiles that fit more on screen for dense dashboards).
 - **Top navigation labels** shows text labels beside the top navigation icons. Turn it off for an icon-only bar; the destinations stay reachable by hover tooltip, accessible name, and the command palette. On the phone layout the navigation always keeps its labels. With labels off, **Top navigation alignment** chooses whether the icon-only bar sits to the left or centered.
-- **Deploy progress** toggles live output streaming for deploy, restart, update, install, and Git operations, and **Progress style** chooses Modal or Inline presentation.
-- **Diff preview before save** toggles a side-by-side diff of compose and env edits before they are written to disk.
+
+Deploy-progress behavior and the diff-preview-before-save step are stack workflow preferences, so they live in **Settings → Infrastructure → Stacks**, not here.