feat: typed SSE client primitive (port from Teller, .ts)

[constk]

Problem

Streaming-LLM apps need an SSE client. Hand-written SSE parsing is a known footgun (RFC 6455-style event:/data:/blank-line state machine). Shipping a tested, typed primitive saves users that footgun.

Proposed solution

Port frontend/src/lib/api/client.js from Teller into frontend/src/lib/api/client.ts. Type the event shapes with discriminated unions. Keep the parser pure (no DOM dependency). Add a vitest unit test for the parser using a fake ReadableStream.

Acceptance criteria

• client.ts exports a typed consumeSSE function returning an AsyncIterable of typed events.
• Vitest unit test covers: event:/data: parsing, multi-line data:, blank-line dispatch, partial-chunk reassembly.
• Strict TS passes; eslint zero warnings.

Priority rationale

High: this is one of the most copy-pasted-incorrectly primitives in LLM-frontend code. Shipping it correctly is a portfolio differentiator.

Depends on

#21

[constk]

Merged in PR #59.