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.

docs/design.md

@@ -30,7 +30,7 @@ Before adding a new component, read `components/ui/`. The primitive usually exis
 
 Hierarchy is conveyed through weight and color, not size. Most interface text is `fontSize.base`; compact metadata and hints use `fontSize.sm`. The distinction between a row's primary line and its secondary line is `foreground` versus `foregroundMuted`.
 
-The authored ramp uses a 14px base. New native installs default to 15px; web and desktop default to 14px. The Appearance **Base size** setting is the rendered `fontSize.base` value and scales the rest of the UI ramp proportionally. Code size remains independent.
+The authored interface ramp uses a 14px base. New native installs default to 15px; web and desktop default to 14px. The Appearance **Interface size** setting is the rendered `fontSize.base` value and scales the rest of the UI ramp proportionally. Primary readable content has its own `fontSize.content`, which defaults to 15px on every platform. It owns message bodies, composer input, Markdown, and PR prose. Controls, navigation, metadata, tool chrome, code, diffs, editors, and terminals stay on their interface or code tokens. **Code size** remains independent.
 
 Weight has three tiers, applied by role:
 

docs/unistyles.md

@@ -344,7 +344,7 @@ Gotchas:
 - **Patch all themes, not just the active one.** The active theme can change and adaptive mode can flip light/dark; patching every key keeps the active key current and makes ordering vs `setTheme`/`setAdaptiveThemes` irrelevant. The effect depends on the settings values (not on `theme`), so it cannot loop.
 - **The reserved plugin keys are the exception to that ordering.** A plugin-contributed theme is rebuilt from its palette, which discards the appearance patch, so the appearance provider writes the matching light or dark slot before applying font and syntax preferences. See [plugins.md](plugins.md).
 - **Narrow the discriminated union before spreading.** `updateTheme`'s updater returns the theme union; spreading the union widens `colorScheme` to `"light" | "dark"`, which is assignable to neither concrete member. Branch on `t.colorScheme` so each branch spreads a single narrowed theme type (no `as`).
-- **`lineHeight.diff` is the code/diff line-height axis** — it is coupled to the code-font-size control (≈ `codeFontSize * 1.5`). Do NOT use it for prose. Markdown body line-height scales with the UI ramp (`Math.round(theme.fontSize.base * 1.4)`); routing prose through `lineHeight.diff` clips text at small code sizes.
+- **`lineHeight.diff` is the code/diff line-height axis** — it is coupled to the code-font-size control (≈ `codeFontSize * 1.5`). Do NOT use it for prose. Markdown body line-height scales with content size (`Math.round(theme.fontSize.content * 1.4)`); routing prose through `lineHeight.diff` clips text at small code sizes.
 - **High-churn draft values** (live-while-typing in the appearance preview) bypass the theme: apply them as inline styles marked with `inlineUnistylesStyle` so per-keystroke values don't grow the `#unistyles-web` CSS registry.
 - **The app shell uses one `AppearanceStyleBoundary`.** Runtime-patched numeric theme values are baked into Unistyles web classes rather than CSS variables, while parsed/memoized content also does not naturally re-run when appearance tokens change. The boundary sits below stable runtime providers in `app/_layout.tsx` and remounts the visual shell once. `applyAppearance` patches the active theme before inactive registry entries so its subscribers receive the committed values in the same update. Do not add local appearance keys or nested boundaries.
 - **Dynamic font tokens stay widened.** `fontFamily`, `fontSize`, and `lineHeight` on `commonTheme` are annotated `string`/`number` (not narrowed by `as const`) so the updater's return assigns; the platform default stacks live in `DEFAULT_UI_FONT_STACK` / `DEFAULT_MONO_FONT_STACK`.