Refactor duplicated code patterns into shared utilities

[devin-ai-integration]

Summary

Three code patterns were duplicated across the frontend. This PR extracts each into a small, tested shared utility under src/utils/ and updates all call sites. Net ~140 fewer lines, no behavior change.

1. Error normalization — the idiom err instanceof Error ? err.message : String(err) appeared 38× across 20 files (stores, previews, chat, skills, etc.).

// src/utils/errors.ts
export function getErrorMessage(err: unknown, fallback?: string): string {
  return err instanceof Error ? err.message : (fallback ?? String(err))
}

The optional fallback preserves the variants that used a custom default instead of String(err) (e.g. getErrorMessage(err, t('skills.zipParseError'))).

2. SSE stream parsing — the byte-level event:/data: parser loop was copy-pasted three times (piClient.prompt, piClient.runGoal, goalStore.startGoal). Extracted verbatim into:

// src/utils/sse.ts
export async function readSSEStream(
  reader: ReadableStreamDefaultReader<Uint8Array>,
  onEvent: (eventType: string, data: any) => boolean | void, // return true to stop early
): Promise<void>

Each caller keeps its own no-body handling and try/catch (their behaviors differ) and just supplies an onEvent handler, returning true on the terminal done event. The buffering/line-splitting/JSON-parse semantics are unchanged (including the existing limitation that an event: line and its data: line must arrive in the same buffer).

3. JSON request init — the { method, headers: { 'Content-Type': 'application/json' }, body: JSON.stringify(x) } boilerplate appeared 18×.

// src/utils/http.ts
export function jsonRequest(body: unknown, init?: RequestInit): RequestInit
// fetch(url, jsonRequest(data))                        -> POST
// fetch(url, jsonRequest(body, { method: 'PUT' }))     -> PUT
// fetch(url, jsonRequest(body, { signal }))            -> POST + abort signal

Tests

Added tests/errors.test.ts, tests/http.test.ts, tests/sse.test.ts. Full suite: 41 passing; tsc --noEmit clean.

Link to Devin session: https://app.devin.ai/sessions/96bdc26167854eee8f85a0ce5742c714
Requested by: @JeffGuo7

[devin-ai-integration]

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