Phase 1.5c: Tailwind vendored compile (replace CDN)

[nextlevelshit]

Part of Epic #1565 Phase 1.5.

Goal

Replace Tailwind CDN `<script src="https://cdn.tailwindcss.com">` with a vendored compiled stylesheet. Avoids runtime CDN dep + offline-mode break before Phase D ship.

Acceptance criteria

• `tailwindcss` CLI run at build time, output to `internal/webui/static/tailwind.css`
• Stylesheet embedded via `//go:embed`
• Templates reference embedded path, not CDN
• Build step documented in Makefile / docs/build.md
• No external CDN dependency at runtime

Dependencies

• 1.5a Phase A (PR feat(webui): /preview/* phase A (build tag, fixtures only) #1585 MERGED) — uses CDN currently
• Optionally after 1.5b

[nextlevelshit]

Closed via PR #1604 MERGED — Tailwind vendored.