feat(qol): error boundaries + rate-limit aware fetching

[dPeluChe]

The two quality-of-life picks from the backlog.

🛟 Error boundaries

• New ErrorBoundary (friendly fallback: error message + Try again / Reload app).
• App-level on the root route, per-view around the Home main column — keyed by scope/repo so navigating away auto-resets a crashed view, and the sidebar lives outside the boundary: a broken scope no longer blanks the app, you just switch scopes.
• The standalone /repos/:owner/:name route gets its own.

⏳ Rate-limit aware fetching (v1)

• store/rateGate.ts — passive quota tracking: gql()/rest() report the x-ratelimit-* headers every response already carries (zero extra API calls), plus the dedicated rateLimit query feeds the graphql pool. The volatile per-minute search pool is deliberately ignored (it self-heals; gating on it would cause false alarms).
• When a tracked pool drops under 5% (min 100): react-query's focus/reconnect refetching pauses — manual actions and initial loads unaffected — and the gate self-lifts at the reset timestamp.
• Dashboard topbar: the quota readout turns red with ⚠ and a "background refresh paused until " tooltip.

Tests

6 new (thresholds, header parsing, search-pool ignore, reset behavior) → suite is now 35 tests / 8 files. npm run check green.

Verify

• Throw inside any scope (devtools) → fallback card, sidebar still works, switching scopes recovers.
• Topbar quota readout: normal now; goes ⚠ red under 5%.

Note: branched from main — independent of the still-open #36 (no file overlap).

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