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/editor.mdx

@@ -127,7 +127,7 @@ The same controls apply to the `compose.yaml` and `.env` editors.
 
 ## Diff preview before save
 
-When **Diff preview before save** is enabled in **Settings → Appearance**, clicking **Save & Deploy** or **Save Only** opens a side-by-side diff modal before anything is written to disk. The left pane is the on-disk content; the right pane is your unsaved edits with additions highlighted in green. The footer reads `ON DISK → UNSAVED` so the panes are unambiguous.
+When **Diff preview before save** is enabled in **Settings → Infrastructure → Stacks**, clicking **Save & Deploy** or **Save Only** opens a side-by-side diff modal before anything is written to disk. The left pane is the on-disk content; the right pane is your unsaved edits with additions highlighted in green. The footer reads `ON DISK → UNSAVED` so the panes are unambiguous.
 
 <Frame>
   <img src="/images/compose-diff-preview/diff-modal.png" alt="Diff preview modal showing side-by-side YAML diff with the unsaved version on the right and an ON DISK to UNSAVED legend in the footer" />