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

packages/app/src/constants/theme.ts

@@ -3,8 +3,6 @@
  * There are many other ways to style your app. For example, [Nativewind](https://www.nativewind.dev/), [Tamagui](https://tamagui.dev/), [unistyles](https://reactnativeunistyles.vercel.app), etc.
  */
 
-import { Platform } from "react-native";
-
 const tintColorLight = "#0a7ea4";
 const tintColorDark = "#fff";
 
@@ -26,28 +24,3 @@ export const Colors = {
     tabIconSelected: tintColorDark,
   },
 };
-
-export const Fonts = Platform.select({
-  ios: {
-    /** iOS `UIFontDescriptorSystemDesignDefault` */
-    sans: "system-ui",
-    /** iOS `UIFontDescriptorSystemDesignSerif` */
-    serif: "ui-serif",
-    /** iOS `UIFontDescriptorSystemDesignRounded` */
-    rounded: "ui-rounded",
-    /** iOS `UIFontDescriptorSystemDesignMonospaced` */
-    mono: "ui-monospace",
-  },
-  default: {
-    sans: "normal",
-    serif: "serif",
-    rounded: "normal",
-    mono: "monospace",
-  },
-  web: {
-    sans: "system-ui, -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, Helvetica, Arial, sans-serif",
-    serif: "Georgia, 'Times New Roman', serif",
-    rounded: "'SF Pro Rounded', 'Hiragino Maru Gothic ProN', Meiryo, 'MS PGothic', sans-serif",
-    mono: "SFMono-Regular, Menlo, Monaco, Consolas, 'Liberation Mono', 'Courier New', monospace",
-  },
-});