feat(menubar): customizable metric widgets in the status item (#82)

[caezium]

What

Satisfies #82 — let users choose which metrics appear in the menu bar and how each is displayed, the way a desktop system monitor does. Replaces the single fixed " 42% · 5.2G" string in Metrics mode with a configurable, reorderable row of metric widgets.

Re-implemented in Burrow's own MIT Swift + Brand design system (no GPL code copied; the look/UX follows the common menu-bar-monitor conventions).

Widget styles

• Value — 42%
• Label + value — CPU 42%
• Bar — mini fill bar + value (utilization-coloured)
• Sparkline — mini line chart (net/disk off the 1 Hz ring; CPU/RAM/GPU off a snapshot-cadence ring)
• Speed ↓↑ — two-line down/up rate (network & disk I/O)
• Battery glyph — battery icon + % with charging state

Metrics: CPU, RAM, GPU, disk used, network, disk I/O, fan, temperature, battery. Each offers only the styles that make sense for it; colour mode is per-widget (by utilization / accent / monochrome / by pressure).

How it's wired

• Store.menuBarItems — ordered [MenuBarItem] (metric + style + colour), JSON-persisted. Default = CPU + memory, and Metrics mode stays opt-in (default is the icon) → no change for existing users until they customize.
• MenuBarWidgets.swift (new) — the model + MenuBarRenderer, which draws the row into an NSImage via a drawing handler so monochrome text re-resolves for light/dark menu bars. The status button keeps its existing click/right-click action untouched (no custom hit-testing view).
• StatusBarController builds the values from the live feed (a snapshot sink + a 1 Hz sink) and re-renders. Drawing is a few strings/shapes and reads only already-published live values — no NSWorkspace/blocking work on the main thread (consistent with the hang fixes in fix(perf): eliminate the main-thread App Hangs (Sentry triage) #83).
• SettingsView ▸ Menu Bar — an editor (add / remove / reorder, per-metric style + colour pickers) shown when Display = Metrics; edits persist + re-render the status item live.

Test plan

• xcodebuild Debug build succeeds, no new warnings.
• Settings ▸ Menu Bar → Display = Metrics: the configured row appears in the status bar.
• Add / remove / reorder metrics; switch styles (value / label / bar / sparkline / speed / battery) and colour modes — confirm each renders and updates live.
• Check legibility on a light and a dark menu bar.
• Confirm net/disk speed + sparkline animate at ~1 Hz; CPU/RAM update at the sample cadence.
• Left-click still opens the popover; right-click still shows the quick menu.

Notes

• Companion to fix(perf): eliminate the main-thread App Hangs (Sentry triage) #83 (the Sentry hang fixes). Independent branch off origin/main; no file overlap, so the two can land in either order.
• Closes 关于菜单栏可以自定义指标的建议 #82.