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/highlight/src/__tests__/themes.test.ts

@@ -0,0 +1,81 @@
+import { describe, it, expect } from "vitest";
+import { darkHighlightColors, lightHighlightColors } from "../colors.js";
+import { SYNTAX_THEME_IDS, isSyntaxThemeId, resolveSyntaxColors } from "../themes.js";
+import type { HighlightStyle } from "../types.js";
+
+const allStyles: HighlightStyle[] = [
+  "keyword",
+  "comment",
+  "string",
+  "number",
+  "literal",
+  "function",
+  "definition",
+  "class",
+  "type",
+  "tag",
+  "attribute",
+  "property",
+  "variable",
+  "operator",
+  "punctuation",
+  "regexp",
+  "escape",
+  "meta",
+  "heading",
+  "link",
+];
+
+const colorSchemes: ("light" | "dark")[] = ["light", "dark"];
+
+describe("resolveSyntaxColors", () => {
+  for (const id of SYNTAX_THEME_IDS) {
+    for (const colorScheme of colorSchemes) {
+      describe(`${id} (${colorScheme})`, () => {
+        const colors = resolveSyntaxColors(id, colorScheme);
+
+        it("covers all HighlightStyle values", () => {
+          for (const style of allStyles) {
+            expect(colors[style]).toBeDefined();
+            expect(typeof colors[style]).toBe("string");
+          }
+        });
+
+        it("has valid hex color values", () => {
+          for (const style of allStyles) {
+            expect(colors[style]).toMatch(/^#[0-9a-fA-F]{6}$/);
+          }
+        });
+      });
+    }
+  }
+
+  it("github + light deep-equals lightHighlightColors", () => {
+    expect(resolveSyntaxColors("github", "light")).toEqual(lightHighlightColors);
+  });
+
+  it("github + dark deep-equals darkHighlightColors", () => {
+    expect(resolveSyntaxColors("github", "dark")).toEqual(darkHighlightColors);
+  });
+
+  it("dark-only themes ignore the color scheme", () => {
+    for (const id of ["dracula", "nord"] as const) {
+      expect(resolveSyntaxColors(id, "light")).toEqual(resolveSyntaxColors(id, "dark"));
+    }
+  });
+});
+
+describe("isSyntaxThemeId", () => {
+  it("accepts every id in SYNTAX_THEME_IDS", () => {
+    for (const id of SYNTAX_THEME_IDS) {
+      expect(isSyntaxThemeId(id)).toBe(true);
+    }
+  });
+
+  it("rejects unknown ids", () => {
+    expect(isSyntaxThemeId("auto")).toBe(false);
+    expect(isSyntaxThemeId("github-light")).toBe(false);
+    expect(isSyntaxThemeId("one-dark")).toBe(false);
+    expect(isSyntaxThemeId("nope")).toBe(false);
+  });
+});