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

[jacisko]

What

Adds a ready-to-copy TokyoNight Storm theme file under a new themes/ directory, plus a short README explaining these are examples the app does not auto-load (themes are still read only from the per-machine <userData>/themes/ folder, per docs/themes.md).

Why

The custom-theme system landed recently, but there is no bundled example beyond the runtime-generated _example.json. A curated theme file gives users a real palette to copy, and gives the project a natural home for community/example themes should you want to wire bundled themes into the app later.

Details

• themes/tokyonight-storm.json sets all 21 THEME_COLOR_TOKENS (from src/shared/types.ts), so it renders fully rather than inheriting half its palette from the built-in light/dark defaults.
• Core palette hexes match the official folke/tokyonight.nvim Storm variant (bg #24283b, fg #c0caf5, blue #7aa2f7, cyan #7dcfff, green #9ece6a, yellow #e0af68, red #f7768e). Stem-specific UI tokens with no TokyoNight equivalent (panel, line, hair, sel, inline-bg, accent-ink) are derived from that palette.
• themes/README.md notes that files here are not auto-loaded: copy into the userData themes folder and press Reload.

Notes / open questions

• This does not wire any loading; it is pure data. If you would prefer these live elsewhere (e.g. under docs/), or want a bundled-themes copy-on-first-run mechanism, happy to rework.
• No code touched, so the themes test suite (tests/unit/themes.test.ts, which reads a throwaway STEM_THEMES_DIR) is unaffected.