feat: tsuki Firefox theme for Nova (157+)

Adaptive light/dark static theme generated from @tsuki/tokens, plus an
optional userChrome.css layer (IBM Plex, tsuki radii, shu accent) that
overrides Nova's design tokens. Self-hosted via AMO unlisted signing and
Gitea releases.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

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+# @tsuki/firefox-theme
+
+tsuki for Firefox 157+ (Nova). One adaptive theme: kami (paper) when your system is light, sumi (ink) when it's dark, with shu marking the active tab, loading state, and focused address bar.
+
+Two layers:
+
+- **Theme** (`tsuki-<version>.xpi`) — colors only. Signed by Mozilla, self-hosted on Gitea, updates automatically.
+- **userChrome.css** (optional) — what the theme API can't reach: IBM Plex Sans, tsuki corner radii (4/6/8px instead of Nova's 12–24px), and shu in place of Nova's violet for focus rings, links, and primary buttons.
+
+Both are generated from [`@tsuki/tokens`](https://git.d-v.cc/tsuki/tokens). No hex values are written by hand.
+
+## Install
+
+### Theme
+
+Download `tsuki-<version>.xpi` from the [latest release](https://git.d-v.cc/tsuki/firefox-theme/releases) and open it in Firefox. Firefox checks `updates.json` in this repo for new versions.
+
+### userChrome.css
+
+1. Open `about:config` and set `toolkit.legacyUserProfileCustomizations.stylesheets` to `true`.
+2. Open `about:support` and click **Open Folder** next to *Profile Folder*.
+3. Unzip `tsuki-userchrome-<version>.zip` from the release and copy the contents of its `userChrome/` folder into a `chrome/` folder in your profile, giving you `chrome/userChrome.css` and `chrome/tsuki-fonts/`.
+4. Restart Firefox.
+
+If you already have a `userChrome.css`, paste tsuki's at the top of yours and keep the `tsuki-fonts/` folder next to it.
+
+The overrides target Nova's design tokens (`--border-radius-*`, `--color-accent-primary`, `--focus-outline-color`), not element selectors, so they should survive most Firefox updates. Mozilla doesn't support userChrome.css, though, and a future release may rename these tokens.
+
+## Design notes
+
+- **No gradients.** Nova encourages gradient themes. tsuki doesn't use them, and the tests fail on any `gradient` or `images` in the output.
+- **Selected tab = toolbar.** Both are `kami-0` / `sumi-100`, so the active tab reads as one sheet with the toolbar. Separation comes from color, not shadow.
+- **Dead Nova keys are omitted:** `ntp_text`, `sidebar_highlight`, `sidebar_highlight_text`.
+- **Contrast is tested:** text pairs ≥ 4.5:1, icons and accent marks ≥ 3:1, in both modes.
+
+To change a color, edit [`src/mapping.ts`](src/mapping.ts).
+
+## Develop
+
+```bash
+pnpm install
+pnpm build        # dist/theme, dist/userChrome, updates.json
+pnpm test
+pnpm lint         # web-ext lint (self-hosted rules)
+pnpm start        # Firefox with the theme + userChrome, light
+pnpm start:dark   # same, forced dark
+```
+
+`pnpm start` uses a throwaway profile in `.preview-profile/`, not your own.
+
+### Test checklist (Mozilla's for Nova)
+
+Horizontal and vertical tabs · sidebar closed / open / expand-on-hover · compact density · private window · light and dark · hover states on tabs and toolbar buttons.
+
+## Releasing
+
+Release Firefox only installs Mozilla-signed add-ons, so "self-hosted" means signed through AMO's **unlisted** channel (not published on addons.mozilla.org), then distributed from Gitea.
+
+One-time setup:
+
+1. Create AMO API credentials at <https://addons.mozilla.org/developers/addon/api/key/>.
+2. Add repo secrets: `AMO_JWT_ISSUER`, `AMO_JWT_SECRET`, `NPM_TOKEN` (registry read), `RELEASE_TOKEN` (Gitea token with repo write).
+3. The repo must be public: Firefox fetches `updates.json` and the XPI without auth.
+
+Cut a release:
+
+```bash
+npm version <patch|minor|major> --no-git-tag-version
+pnpm build                       # regenerates updates.json for the new version
+git commit -am "release: vX.Y.Z"
+git tag vX.Y.Z
+git push --follow-tags
+```
+
+CI verifies `updates.json` is committed for the tag, signs the XPI, and attaches it and the userChrome zip to a Gitea release.