fix(github): attribute GitHub API outages instead of blank/"failed" states

When GitHub's API is unreachable (5xx outage, network, or rate limit), Orca
showed no PR data with no explanation, so it read as an Orca bug rather than a
GitHub-side problem.

- Add a shared classifier (classifyGitHubUnavailable) reused by the main
  process and the renderer so every surface attributes an outage identically.
  A live outage returns HTTP 5xx, which the PR-refresh classifier previously
  had no branch for (fell through to the un-attributed "refresh failed").
- Right-sidebar Checks panel: show GitHub-attributed copy in the error
  empty-state, plus an inline banner over stale cached PR data so an outage
  doesn't look like a normal (silently out-of-date) panel.
- Tasks/PR-list page: replace the vague "N of M projects failed to load" with
  a GitHub-attributed banner when the failure is a reachability problem.

Copy names GitHub as the source and reassures it isn't an Orca problem, with no
status-page link. Stays GitHub-scoped so GitLab/other providers aren't
mislabeled.

src/shared/github-api-availability.test.ts

@@ -0,0 +1,58 @@
+import { describe, expect, it } from 'vitest'
+import { classifyGitHubUnavailable, isGitHubUnavailableError } from './github-api-availability'
+
+describe('classifyGitHubUnavailable', () => {
+  it('classifies HTTP 5xx outages as server_error', () => {
+    expect(classifyGitHubUnavailable('HTTP 503: Service Unavailable')).toBe('server_error')
+    expect(classifyGitHubUnavailable('gh: Command failed: HTTP 502 Bad Gateway')).toBe(
+      'server_error'
+    )
+    expect(classifyGitHubUnavailable('GitHub API error: 500 Internal Server Error')).toBe(
+      'server_error'
+    )
+    expect(classifyGitHubUnavailable('The service is temporarily unavailable')).toBe('server_error')
+  })
+
+  it('classifies transport failures as network', () => {
+    for (const message of [
+      'request to https://api.github.com failed, reason: getaddrinfo ENOTFOUND api.github.com',
+      'dial tcp: lookup api.github.com: no such host',
+      'connect ETIMEDOUT 140.82.112.5:443',
+      'fetch failed',
+      'socket hang up',
+      'could not resolve host: api.github.com',
+      'connection refused'
+    ]) {
+      expect(classifyGitHubUnavailable(message)).toBe('network')
+    }
+  })
+
+  it('classifies rate limiting as rate_limited (even when it carries HTTP 403)', () => {
+    expect(classifyGitHubUnavailable('HTTP 403: API rate limit exceeded')).toBe('rate_limited')
+    expect(classifyGitHubUnavailable('You have exceeded a secondary rate limit')).toBe(
+      'rate_limited'
+    )
+    expect(classifyGitHubUnavailable('HTTP 429 Too Many Requests')).toBe('rate_limited')
+  })
+
+  it('returns null for non-reachability failures', () => {
+    expect(classifyGitHubUnavailable('HTTP 403: Resource not accessible by integration')).toBeNull()
+    expect(classifyGitHubUnavailable('HTTP 404: Not Found')).toBeNull()
+    expect(classifyGitHubUnavailable('could not resolve to a Repository with the name')).toBeNull()
+    expect(classifyGitHubUnavailable('gh auth login required')).toBeNull()
+    expect(classifyGitHubUnavailable('')).toBeNull()
+  })
+
+  it('does not misread unrelated 3-digit numbers as a server outage', () => {
+    expect(classifyGitHubUnavailable('found 512 pull requests')).toBeNull()
+  })
+})
+
+describe('isGitHubUnavailableError', () => {
+  it('detects reachability failures from Error objects and strings', () => {
+    expect(isGitHubUnavailableError(new Error('HTTP 503: Service Unavailable'))).toBe(true)
+    expect(isGitHubUnavailableError('fetch failed')).toBe(true)
+    expect(isGitHubUnavailableError(new Error('HTTP 404: Not Found'))).toBe(false)
+    expect(isGitHubUnavailableError(null)).toBe(false)
+  })
+})