H4 + H2 formatter half: carry raw numbers across the progress boundary

Closes H4 and the remaining formatter half of H2.

- New @shared/format.ts is the single home for fmtBytes/fmtSpeed/fmtEta, imported
  by both main and renderer. main/lib/formatters.ts re-exports them.
- DownloadProgress now carries raw speedBytesPerSec/etaSeconds instead of
  pre-formatted speed/eta strings. main's parseProgress emits the raw numbers.
- The renderer stores the raw numbers on DownloadItem; QueueItem formats them for
  per-item display, and summarizeQueue sums/maxes them directly. The lossy
  string->number->string round-trip in queueStats (parseSpeed / parseEtaSeconds /
  a local formatSpeed / formatEta) is deleted, along with a duplicate fmtEta in
  store/downloads.ts.
- Per-item and aggregate speed now use the same 1024-based scale; the aggregate
  previously used a 1000-based formatter (a latent inconsistency).

Tests updated for the raw-number contract (parseProgress, summarizeQueue).
typecheck + 248 tests + eslint + prettier green.

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

test/queueStats.test.ts

@@ -48,19 +48,25 @@ describe('summarizeQueue', () => {
     expect(s.progress).toBeCloseTo(0.4, 5)
   })
 
-  it('sums download speeds across MB/s and MiB/s strings', () => {
+  it('sums raw download rates into one formatted label', () => {
+    const MB = 1024 * 1024
     const s = summarizeQueue([
-      item({ status: 'downloading', progress: 0.1, speed: '4.0 MB/s' }),
-      item({ status: 'downloading', progress: 0.1, speed: '2000 KiB/s' })
+      item({ status: 'downloading', progress: 0.1, speedBytesPerSec: 4 * MB }),
+      item({ status: 'downloading', progress: 0.1, speedBytesPerSec: 2 * MB })
     ])
-    // 4.0 MB/s + ~2.0 MB/s ≈ 6.0 MB/s
+    // 4 MB/s + 2 MB/s = 6 MB/s
     expect(s.speedLabel).toBe('6.0 MB/s')
   })
 
+  it('leaves the speed label empty when no item reports a rate', () => {
+    const s = summarizeQueue([item({ status: 'downloading', progress: 0.1 })])
+    expect(s.speedLabel).toBe('')
+  })
+
   it('reports the longest active ETA as the time left', () => {
     const s = summarizeQueue([
-      item({ status: 'downloading', progress: 0.5, eta: '0:30' }),
-      item({ status: 'downloading', progress: 0.2, eta: '2:10' })
+      item({ status: 'downloading', progress: 0.5, etaSeconds: 30 }),
+      item({ status: 'downloading', progress: 0.2, etaSeconds: 130 })
     ])
     expect(s.etaLabel).toBe('2:10')
   })