feat(battery): show charge/discharge rate in the bar widget

[SpyrosPsarras]

Description

The battery popout already shows the UPower energy rate, but the bar widget could only show percentage and remaining time. Telling how fast the battery is charging or draining meant opening the popout, or running a third-party plugin that shells out to /sys/class/power_supply/BAT*/power_now on a timer.

This adds two independent, opt-in toggles to the battery widget — Show Charge Rate and Show Discharge Rate — which render the signed rate alongside the existing text.

Charging
Discharging (with percentage + remaining time also enabled)

Design notes

• No new process or polling. The value is BatteryService.changeRate, the same aggregate the popout already renders.
• Costs no bar space unless it has something to say. Below 0.05W — idle, fully charged, or parked at a charge limit — the text is empty and the widget collapses back to its previous width.
• Stays narrow. One decimal below 10W (-8.4W), whole watts above (+28W); vertical bars get a whole-watt line so the value fits the bar thickness.
• Two toggles instead of one + an "Only on Battery" sub-row. Charging and discharging are genuinely separate interests (charger throughput vs. idle drain), and an "only on battery" modifier is meaningless for a discharge readout. This also keeps both labels inside the 270px menu without eliding.
• Follows the existing widget-setting pattern: a global default in SettingsData/SettingsSpec.js plus a per-widget-instance override plumbed through Lists.qml / WidgetsTab.qml / WidgetsTabSection.qml. Both default to false, so nothing changes for existing users until they opt in.

The string composition in Battery.qml was refactored from stacked early-returns to a parts array so a third element could be added without a combinatorial explosion; the output is byte-identical to master for every existing combination of percentage/time toggles.

BatteryService gains two small reusable pieces: signedChangeRate (positive charging, negative draining, zero idle) and formatPowerRate(compact). Happy to follow up by pointing BatteryPopout at them too — it currently derives the sign from isCharging || isPluggedIn, which mislabels a plugged-in-but-draining machine as +. Left out here to keep the diff focused.

Type of change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that changes existing behavior)
• Refactor / internal cleanup
• Documentation
• Other

Related issues

None.

Screenshots / video

Above. Both captured on a live session: charging at 28.3W reported by upower renders +28W; discharging at 15.4W renders -15W.

Checklist

• My code follows the conventions in CONTRIBUTING.md
• I have tested my changes locally
• New user-facing strings are wrapped in I18n.tr() with translator context, reusing existing terms where possible
• Go changes: ran make fmt, added/updated tests, make test passes, and go mod tidy is clean — n/a, no Go changes
• QML changes: ran make lint-qml with no new warnings
• I have opened a corresponding pull request in dlx-docs to document any new behaviors

Testing done: ran the branch against a live niri session and exercised charging, discharging, and the charge-limit/idle case (verified the readout disappears at 0W rather than showing +0W); toggled both settings from the widget context menu and confirmed the per-widget override persists; make lint-qml passes; check_term_variants.py and extract_settings_index.py are clean (the new strings live in a widget context menu, so the settings search index is unchanged); catalogs left alone per CONTRIBUTING.

On the last checkbox — glad to open a dlx-docs PR for the two new toggles if you'd like them documented; just say where they belong.