feat(themes): add TokyoNight Storm as an example theme

themes/README.md

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+# Example themes
+
+Ready-to-use theme files for Stem's custom-theme system (see `docs/themes.md`).
+
+These are **not loaded automatically** — the app only reads themes from this
+machine's themes folder (`<userData>/themes/`). To try one, copy the file there
+(Settings → App → Appearance → **Open themes folder**) and press **Reload**.
+
+Each file sets every color token from `THEME_COLOR_TOKENS` (`src/shared/types.ts`),
+so it renders fully rather than inheriting half its palette from the built-in
+light/dark defaults.
+
+## Themes
+
+- `tokyonight-storm.json` — TokyoNight Storm (dark). Palette from the
+  [folke/tokyonight.nvim](https://github.com/folke/tokyonight.nvim) Storm variant.

themes/tokyonight-storm.json

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+{
+  "name": "TokyoNight Storm",
+  "appearance": "dark",
+  "colors": {
+    "paper": "#24283b",
+    "content": "#1f2335",
+    "panel": "#1d202f",
+    "ink": "#c0caf5",
+    "muted": "#565f89",
+    "line": "#2f3549",
+    "hair": "rgba(192, 202, 245, 0.09)",
+    "accent": "#7aa2f7",
+    "accent-ink": "#24283b",
+    "surface": "#1f2335",
+    "field": "#1f2335",
+    "sel": "rgba(122, 162, 247, 0.22)",
+    "inline-bg": "#2a2e42",
+    "info": "#7dcfff",
+    "warn": "#e0af68",
+    "success": "#9ece6a",
+    "danger": "#f7768e",
+    "code-bg": "#1d202f",
+    "code-ink": "#c0caf5",
+    "drop-chat": "#7aa2f7",
+    "drop-files": "#7dcfff"
+  }
+}