Give readable content its own text size (#3637)

* feat(app): separate content typography from interface size

Readable prose benefits from its own scale without inflating navigation and controls. Preserve existing users’ rendered size during migration while giving fresh desktop installs a 15px content default.

* test(app): cover independent content typography

The appearance journey now verifies that changing interface size leaves content size intact. Timeline traversal yields when a test deliberately holds the requested page, so the helper no longer waits on its own gate.

* test(desktop): cover independent content typography

The desktop appearance regression follows the renamed Interface control and verifies it does not alter Content size.

packages/app/src/i18n/resources/en.ts

@@ -2111,13 +2111,18 @@ export const en = {
         interfaceFont: "Interface font",
         interfaceFontHint: "Used across the app. Leave empty for the system default",
         interfaceFontAccessibility: "Interface font family",
-        baseSize: "Base size",
-        baseSizeAccessibility: "Base font size",
+        interfaceSize: "Interface size",
+        interfaceSizeHint: "Used for navigation, controls, and labels",
+        interfaceSizeAccessibility: "Interface font size",
+        contentSize: "Content size",
+        contentSizeHint: "Used for chat text and rendered Markdown",
+        contentSizeAccessibility: "Content font size",
         codeFont: "Code font",
         codeFontHint:
           "Used in code, diffs, and the terminal output. Leave empty for the system default",
         codeFontAccessibility: "Code font family",
         codeSize: "Code size",
+        codeSizeHint: "Used for code, diffs, and terminal output",
         codeSizeAccessibility: "Code font size",
       },
       syntax: {
@@ -2125,7 +2130,8 @@ export const en = {
         highlightTheme: "Highlight theme",
         highlightThemeHint: "Colors for code, independent of the app theme",
         highlightThemeAccessibility: "Highlight theme: {{value}}",
-        previewAccessibility: "Live preview of the syntax theme and code font",
+        previewAccessibility: "Live preview of content typography, syntax theme, and code font",
+        previewContent: "Readable content and code preview",
       },
     },
     shortcuts: {