Fix C1/C2 + reliability cluster (L140/L141/L148, R6/R7)

Critical:
- C1: single source of truth for IPC mock + Settings defaults. DEFAULT_SETTINGS
  in @shared/ipc; main DEFAULTS, renderer FALLBACK, and preview MOCK_SETTINGS all
  derive from it. Whole browser mock collapsed into one typed mockApi.ts; main.tsx
  shrinks to window.api = mockApi. PREVIEW check single-sourced in isPreview.ts.
- C2: break the downloads<->sources circular import via a typed event bus
  (store/coordinator.ts). App eagerly imports sources for side-effects so startup
  load + scheduled --sync + downloadCompleted subscription still run.

Reliability:
- L140/L148: cancel/pause release the active slot synchronously; identity-guarded
  releaseActive; per-spawn cookie jar so a same-id retry/resume is never rejected
  by a stale entry nor run over the concurrency cap.
- L141: renderer history capped at 500 + url de-dup to match main.
- R6: writeJsonAtomic reports/logs write failures and keeps data dirty for retry
  instead of an empty catch.
- R7: encryptSecret warns before the plaintext fallback.

typecheck + 243 tests + eslint green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

test/jsonStore.test.ts

@@ -1,4 +1,4 @@
-import { describe, it, expect, afterEach } from 'vitest'
+import { describe, it, expect, afterEach, vi } from 'vitest'
 import { mkdtempSync, rmSync, writeFileSync, readdirSync } from 'fs'
 import { tmpdir } from 'os'
 import { join } from 'path'
@@ -49,6 +49,24 @@ describe('jsonStore atomic write + safe read', () => {
     expect(readJsonArraySafe(file, isRow)).toEqual([{ id: 'a' }, { id: 'c' }])
   })
 
+  it('reports success/failure instead of swallowing a bad write (R6)', () => {
+    const d = tmp()
+    const err = vi.spyOn(console, 'error').mockImplementation(() => {})
+    try {
+      // A normal write lands and reports true, with nothing logged.
+      expect(writeJsonAtomic(join(d, 'data.json'), [{ id: 'a' }])).toBe(true)
+      expect(err).not.toHaveBeenCalled()
+      // A target under a non-existent directory can't be written — it must report
+      // false (so a caller can warn/retry) and log, not throw or silently drop it.
+      expect(writeJsonAtomic(join(d, 'no', 'such', 'dir', 'data.json'), [{ id: 'a' }])).toBe(false)
+      expect(err).toHaveBeenCalledOnce()
+    } finally {
+      err.mockRestore()
+    }
+    // No stray .tmp left behind by the failed write.
+    expect(readdirSync(d)).toEqual(['data.json'])
+  })
+
   it('backs up a corrupt file instead of silently wiping it (R2)', () => {
     const d = tmp()
     const file = join(d, 'data.json')