feat(widget): expand Storybook coverage, theme switcher, and Pages deploy

[PAMulligan]

Summary

Closes #43. Builds on the base Storybook setup from #82 to cover every
meaningful component state, add a theme/direction switcher, and publish
Storybook to Cloudflare Pages.

Acceptance criteria

• pnpm storybook runs locally with sensible defaults
• Stories for ChatWidget, ChatMessage (user/bot/markdown/sources), ChatInput, Header, ErrorBanner, TypingIndicator, GreetingBubble — plus ChatWindow
• Theme switcher (light/dark/auto) configured
• Storybook built and deployed to Cloudflare Pages on every push to main
• Hosted Storybook linked from README and docs site

What changed

Stories — every widget state from the issue: closed bubble & open panel (ChatWidget); empty / conversation / long-list / loading / error / mobile bottom-sheet (ChatWindow); user / bot / markdown / with-sources (ChatMessage); default / loading / near-limit / at-limit (ChatInput); Header; ErrorBanner (retryable + non-retryable); TypingIndicator; and GreetingBubble in all four corners + a long message.

Theme & direction switcher — Theme (light/dark/auto) and Direction (LTR/RTL) toolbar globals, matching the existing custom Locale toolbar. A preview decorator drives the widget's own data-claudius-dark mechanism (and resolves auto via prefers-color-scheme), so every story renders across color schemes and writing direction with no per-story wiring — ideal for Chromatic.

Refactor (no behavior change) — Header, ErrorBanner, and TypingIndicator were inline JSX inside ChatWindow; extracted into their own presentational components so each can be storied in isolation. DOM output is byte-identical, so all existing ChatWindow tests pass unchanged.

Deploy — new .github/workflows/storybook.yml mirrors docs.yml: builds on push + PR (path-filtered to widget/**), deploys storybook-static to the claudius-storybook Pages project on push to main / manual dispatch. Removed the now-duplicate Build Storybook step from ci.yml (the dedicated workflow covers it, same pattern as docs).

Docs — README badge + Development link; docs sidebar entry (Plugins & Tools) and a Storybook section in the local-development page.

One-time setup before the deploy goes green

The deploy reuses the existing CLOUDFLARE_API_TOKEN / CLOUDFLARE_ACCOUNT_ID secrets (same as the Docs workflow), but the Pages project must be created once:

wrangler pages project create claudius-storybook

After that, every push to main publishes to https://claudius-storybook.pages.dev (already wired into the README/docs). It can also be run on demand via Actions -> Storybook -> Run workflow.

Note on Storybook version

The issue suggested Storybook 8, but the repo already runs Storybook 10 with the Vite builder (@storybook/react-vite, from #82). I built on the installed version rather than downgrading; the "Vite builder" intent is preserved.

Verification

• pnpm typecheck, pnpm lint, pnpm format:check — clean
• pnpm test — 257/257 pass (extraction is transparent)
• pnpm build — ESM/CJS/IIFE + types; confirmed 0 stories leak into dist
• pnpm build-storybook — builds successfully

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