feat(plugins): add theme contributions

Adds plugin-contributed light and dark app themes, including persistence, host-aware coalescing, fallback behavior, documentation, examples, and browser/iOS QA.

plugin-examples/catppuccin/README.md

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+# Catppuccin plugin example
+
+This example adds **Catppuccin Mocha** to Settings → Appearance. Paseo ships Catppuccin as a
+syntax-highlight theme; this contributes it as an app theme.
+
+A theme is data, so the whole plugin is one `addTheme` call in `index.ts` — no client file, no RPC.
+
+Register it in `$PASEO_HOME/config.json`:
+
+```json
+{
+  "pluginsEnabled": true,
+  "plugins": {
+    "catppuccin": {
+      "source": "directory",
+      "path": "/absolute/path/to/paseo/plugin-examples/catppuccin"
+    }
+  }
+}
+```
+
+Then run `paseo reload` and pick **Catppuccin Mocha** in Settings → Appearance.
+
+The colors come straight from the [Catppuccin Mocha](https://catppuccin.com/palette/) palette:
+`base`, `text`, `surface0`, `surface1`, `mauve`, `subtext0`, and `overlay0`. Paseo expands them
+into the full token set, so `accent` (`mauve`) drives buttons and selection while `border`
+(`surface1`, shared with `control`) stays the border and raised-surface tint.

plugin-examples/catppuccin/index.ts

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+import type { PluginContext } from "@getpaseo/plugin";
+
+export default function contribute(plugin: PluginContext) {
+  plugin.addTheme({
+    id: "mocha",
+    name: "Catppuccin Mocha",
+    appearance: "dark",
+    colors: {
+      background: "#1e1e2e",
+      foreground: "#cdd6f4",
+      raised: "#313244",
+      control: "#45475a",
+      border: "#45475a",
+      accent: "#cba6f7",
+      mutedForeground: "#a6adc8",
+      ring: "#6c7086",
+    },
+  });
+  return () => {};
+}

plugin-examples/catppuccin/paseo-plugin.json

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+{
+  "id": "catppuccin"
+}