All-devices dashboard tab: surface two-way multi-device sync (#100 FE)

[0bserver07]

The frontend for #100 Phase 2 (two-way multi-device sync) — surfaces the merged cross-device view the backend already ships. FE-only, additive, gated behind sync being configured so the default tabs stay byte-identical.

What's in it

A dedicated beta "Devices" tab (Worktrees/Context-Replay precedent — no change to Overview/Cost render paths):

• This device ↔ All devices scope toggle. Hidden when sync is unconfigured (clean empty state pointing at stackunderflow sync init).
• All-devices view reads GET /api/sync/overview?scope=all-devices: headline merged totals (cost/tokens/sessions), a by-day trend (house Recharts chrome), a per-project breakdown, the contributing peer devices list (alias, last-seen), and a merge_warnings badge when > 0. GET /api/sync/status gates everything.
• services/sync.ts + additive sync types (discriminated SyncOverview union) + 6 node --test cases.

Verification

• tsc --noEmit clean · node --test tests/services/*.test.ts 179 pass · vite build clean (Devices tab bundled) · full Python suite 4083 passed (no backend touched)
• 6 UI files only; no Python/pyproject/version changes

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