fix(dashboard): aggregate paired-device usage in the desktop Dashboard and menubar badge

[marcreynolds]

Summary

Fixes #795. The desktop app's Dashboard showed only the local device's
cost, while the menu bar statistics and the web GUI reported usage
aggregated across paired devices. This makes the desktop Dashboard consistent
with those surfaces by adding a Local / Combined scope control, mirroring
the macOS menu bar's existing Scope setting.

Root cause: the desktop spawned codeburn status --format menubar-json
without --scope combined, so the CLI never ran its paired-device
aggregation and the renderer only ever had local data.

What changed

Desktop app

• New Scope: Local / Combined setting in Settings › General (persists to
  localStorage, default Local), mirroring the menu bar.
• Combined passes --scope combined to the CLI, forces the provider filter to
  all, and clears the Claude-config scope (the CLI rejects --scope combined
  alongside a provider/project filter).
• The Overview hero shows the paired-device aggregate (cost / calls /
  sessions) with a "Combined · N devices" caption and a per-device
  breakdown when Combined is selected.

Menu bar

• The badge figure now reflects Combined scope instead of always showing the
  local total: it fetches the combined payload for the badge's period and
  renders the cross-device aggregate, falling back to local when no combined
  payload is available (cold cache, or an unreachable peer).

Scope / limitations

• Only the hero KPIs aggregate; the detailed panels (daily chart, model table,
  etc.) remain local, because the combined payload carries totals only — this
  matches the menu bar's behavior.
• Aggregation is peer-to-peer over the LAN: a paired device contributes only
  while it is reachable. An unreachable peer degrades gracefully to the
  reachable subset.

Testing

• Desktop: npm run typecheck and npm run build:electron clean; npm test → 445 passing (+9 new covering the scope argv, the Settings control,
  and the combined hero).
• Menu bar: swift build clean; new AppStore scope tests added (run
  under the Xcode toolchain: swift test).
• End-to-end: verified against the live CLI on a real two-device setup —
  Combined reported the aggregate ($163.54 month) vs local-only ($95.94),
  matching the menu bar and web GUI.