Add appearance settings for theme, fonts, and syntax highlighting (#1236)

* Add appearance settings for theme, fonts, and syntax highlighting

New Appearance section in Settings to configure the app theme, UI and
mono font family + size, and a syntax-highlighting theme chosen
independently of light/dark. Font family fields default to empty and
show the active system stack as the placeholder; a live split-diff
preview reflects the choices as they change. The theme picker moves
here from General.

Preferences are client-only (AsyncStorage) and applied at runtime by
patching every registered Unistyles theme via
UnistylesRuntime.updateTheme, so existing StyleSheet consumers repaint
with no per-component preference reads and no useUnistyles. No
daemon/protocol change.

* Expand syntax themes and move the preview under the chooser

Replace the small fixed syntax-theme set with eight popular themes — GitHub,
Catppuccin, Dracula, Tokyo Night, One, Nord, Gruvbox, Solarized — built from a
compact per-theme role palette. The app theme and syntax theme stay separate;
the only coupling is the light/dark axis: a theme with both variants uses its
light palette on a light app and its dark palette on a dark app, while the code
frame (gutter, line numbers, background) keeps following the app theme. Default
is now GitHub (GitHub keeps its existing hand-tuned maps for an exact match).

Move the live preview directly under the syntax-theme chooser, drop the
"preview.ts" filename header, and use a clearer code snippet for it.

* Update settings e2e for the moved theme picker and Appearance section

The theme picker moved from General to the new Appearance section, so the
General-content assertion no longer finds "Theme" — point it at "Default send"
(which stays in General) and add Appearance to the e2e section map plus an
expectAppearanceContent check, and exercise the Appearance section in the
sidebar-navigation test.

* Render the appearance preview as a unified diff

Switch the live preview from side-by-side columns to a single unified diff:
unchanged context lines plus "-"/"+" rows for the change, matching how diffs
read elsewhere in the app. Same snippet, mono font, syntax colors, and live
code-font draft behavior.

* Fix code font size change shrinking all UI text

applyAppearance scaled the live (already-patched) theme ramp instead of the
authored FONT_SIZE ramp, so it was not idempotent: every appearance change
re-scaled the current sizes, compounding the whole-UI font scale. With any
non-default UI size, changing the code size — or any setting — shrank (or grew)
all app text, and it persisted into the theme. Build the ramp from the canonical
FONT_SIZE ramp each apply (code size still set absolutely), so applies are
idempotent and a code-size change never touches the UI ramp.

* Apply the interface font across the app on web

react-native-web stamps a default font onto every text element, so the interface
font only reached the few components that set a font explicitly. Inject one rule
that points all text at a CSS variable (updated live as the setting changes),
with high specificity (1,2,0) so it deterministically beats RN-web's base font
and Unistyles' generated classes — no stylesheet-order fragility. Code, diff, and
monospace surfaces carry a `data-pmono` marker (and their subtrees are excluded),
so they keep their code font.

Web-only: React Native has no global font cascade, so native still applies the UI
font only where components opt in. Inline `code` within chat prose is a known
minor gap (its render paths aren't tagged yet).

* Keep preview code in the code font

* Apply code font settings to terminals

* Fix settings host picker import after rebase

* Format terminal webview bundle

docs/unistyles.md

@@ -289,6 +289,20 @@ That brief transition is expected with the current storage model. It makes track
 
 If we ever need to avoid the transition entirely, store at least the theme preference in synchronous storage and configure Unistyles with `initialTheme`.
 
+## Runtime Theme Patching For User Preferences
+
+Appearance settings (UI/mono font family, font sizes, syntax-highlight theme) are applied by patching every registered theme at runtime with `UnistylesRuntime.updateTheme(name, updater)` — not by threading preference reads through components. `applyAppearance` in `packages/app/src/screens/settings/appearance/apply-appearance.ts` runs from a `ProvidersWrapper` effect on settings load/change and loops all six theme keys, returning `{ ...theme, fontFamily, fontSize, lineHeight, colors.syntax }`.
+
+This works without `useUnistyles()` because every consumer already reads these tokens through `StyleSheet.create((theme) => …)` (or the `withUnistyles`/`uniProps` path for the markdown renderer), so patching the theme repaints tracked views through the native ShadowRegistry with no React re-render.
+
+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.
+- **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.
+- **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.
+- **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`.
+
 ## Debugging
 
 To inspect what the Babel plugin sees, temporarily enable [`debug: true`](https://www.unistyl.es/v3/other/babel-plugin#debug) in `packages/app/babel.config.js`: