Add nice-scale chart axes, dual-axis ping chart with loss overlay, and auth-gated period selector

[fl0w1nd]

TL;DR

Charts had ugly default ticks (e.g. 333.33ms), no loss visualization, and no way to view 7d/30d history. This PR adds a d3-style nice-scale engine for clean axis ticks, dual-axis ping charts (delay + loss overlay), period selection with auth gating, and traffic-limit progress bars on server cards.

Files to review

File	Why
src/components/detail/chartScale.ts	New — core scale engine: nice step, auto-baseline, loss/delay/speed presets, axis-width budgeting
src/components/detail/PingChart.tsx	Major rewrite: LineChart → ComposedChart, dual Y-axes, loss area, period selector, custom tooltip
src/lib/auth.ts	New — useIsLogin() hook, probes /api/v1/profile to gate 7d/30d periods
src/components/ServerCard.tsx	Traffic limit progress bar, billing badge redesign, bidirectional traffic
src/components/detail/HistoryChart.tsx	Nice-scale Y-axis per metric type (percent/speed/raw), unit label in header
src/components/detail/RealtimeCharts.tsx	Nice-scale Y-axis for CPU/memory (fixed 0-100) and speed (dynamic unit)
src/components/detail/ChartTheme.tsx	Edge-aware X-axis tick component (EdgeTickX), X_AXIS_PROPS
src/components/Sparkline.tsx	Decoupled viewBox from container size, showDot prop
src/components/OsIcon.tsx	Brand hex colors from simple-icons instead of fill-current
src/pages/ServerDetail.tsx	Truncate long CPU/GPU strings, billing badge style update
src/lib/api.ts	fetchProfile, fetchServerMonitor accepts MonitorPeriod
src/types/nezha.ts	MonitorPeriod type, ProfileResponse interface
src/lib/i18n.tsx	loginToView i18n key (zh/en)
src/index.css	Remove recharts focus outlines
mock/server.mjs	Period-aware monitor data, deterministic failure windows, auth sim

Why

• Ticks: recharts defaults produce unreadable values like 333.33ms; d3-style nice numbers (1/2/5×10^n) always produce clean labels and predictable axis widths.
• Packet loss: the previous ping chart showed delay only; network issues that cause packet loss were invisible. Dual-axis + loss area makes outages immediately visible.
• Periods: official themes support 7d/30d views gated behind login; this theme had no such capability.
• Traffic limits: server cards showed raw transfer bytes; no visual indicator of how close a server is to its traffic quota.

How

• chartScale.ts implements a standalone nice-scale engine: niceStep (sqrt-threshold 1/2/5/10 selector), niceScale (auto-baseline with configurable ratio, min-span guard, headroom), plus domain-specific helpers (PERCENT_SCALE, lossScale, speedScale, delayTickFormatter, axisWidthFor).
• PingChart switches from LineChart to ComposedChart with two Y-axes: left for delay (P95-trimmed + auto-baseline), right for loss (preset-tier scale). Packet loss is computed per-point using the nezha-dash EWMA algorithm when backend packet_loss is absent.
• Auth gating: useIsLogin() polls /api/v1/profile every 5min; 7d/30d buttons are disabled and auto-revert to 1d on logout.
• ServerCard parses trafficVol (e.g. "500GB") into bytes, computes percentage, renders a color-coded 2px progress bar (green → amber → red at 75%/90%).

Reviewer notes

• chartScale.ts is ~280 lines of pure math with no React deps — easy to unit-test if desired.
• The packet-loss algorithm in PingChart.tsx is ported verbatim from hamster1963/nezha-dash; the EWMA smoothing and coefficient-of-variation thresholds match upstream.
• Mock server auth uses document.cookie = "nz-jwt=mock" — documented in a comment in server.mjs.

Visual aids

Before	After
Y-axis: 333.33ms, 666.67ms…	Y-axis: 200ms, 400ms, 600ms
Ping chart: delay lines only	Ping chart: delay lines + loss area overlay
No period selector	1d / 7d / 30d tabs (7d/30d locked for guests)
Card: raw transfer bytes	Card: traffic progress bar with percentage
OS icons: monochrome fill-current	OS icons: brand hex colors