feat: display number of x-runs and show CPU usage

[OpenSauce]

Useful!

closes #193

[Copilot]

Pull request overview

Adds real-time JACK status indicators to the UI by tracking xruns (buffer underruns) and displaying current JACK CPU load in the peak meter/status area, addressing issue #193.

Changes:

• Extend i18n Translations with new labels for xruns and CPU usage (EN + ZH_CN).
• Track xruns via a shared Arc<AtomicU64> updated from jack::NotificationHandler::xrun.
• Plumb xrun count + JACK CPU load into PeakMeterDisplay and render them with basic threshold-based coloring.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/i18n/mod.rs	Adds translatable strings for xruns and cpu.
src/gui/components/peak_meter.rs	Extends the peak meter widget to store/render xrun count and CPU load with colored status text.
src/gui/app.rs	Updates the peak meter polling handler to pass xrun count and CPU load into the display.
src/audio/manager.rs	Stores shared xrun counter, exposes xrun_count()/cpu_load() getters, and wires a stateful notification handler into activate_async.
src/audio/jack.rs	Implements xrun() notification callback to increment the shared xrun counter.

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