feat: add useBattery hook

[samithahansaka]

Summary

• Adds useBattery hook that tracks device battery state using the Battery Status API
• Returns { isSupported, fetched, charging, chargingTime, dischargingTime, level }
• Subscribes to battery change events
• SSR-safe with proper cleanup

Test plan

• DOM tests (8 tests)
• SSR tests (5 tests)
• All 540 project tests pass
• Lint passes
• Build succeeds

Partial implementation of #33 (sensor hooks from react-use)

[Copilot]

Pull request overview

This PR adds a new useBattery hook that tracks device battery state using the Battery Status API. The hook provides battery charge level, charging status, and time estimates, with proper SSR safety and event subscription handling.

Changes:

• Adds useBattery hook with support for tracking battery charging state, level, and time estimates
• Includes comprehensive test coverage with 8 DOM tests and 5 SSR tests
• Adds Battery API mock setup for testing environment

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
src/useBattery/index.ts	Core hook implementation that subscribes to Battery Status API events and returns battery state
src/useBattery/index.dom.test.ts	DOM environment tests covering hook behavior, event subscriptions, and state updates
src/useBattery/index.ssr.test.ts	SSR tests verifying safe fallback behavior when Battery API is unavailable
src/util/testing/setup/battery.test.ts	Test setup file that mocks the Battery Status API for vitest
vitest.config.ts	Configuration update to include battery test setup
src/index.ts	Exports the new useBattery hook under the Navigator section

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[samithahansaka]

I have rebased this onto the latest master. It was out of date and had a conflict, because it changed vitest.config.ts and that file is gone now.

What I changed:

• Moved to the new vp toolchain. The battery test setup is now registered in vite.config.ts.
• Fixed the three review comments from January: cleanup on unmount, handling a failed getBattery() call, and the wording of the fetched doc.
• Added the hook to the README list. I missed that before.
• Put the four event names in one list, so on and off cannot get out of sync.
• Squashed everything into one feat: commit.

Checks on my machine: yarn fmt:check, yarn lint and yarn build all pass, and 547 tests pass. Coverage for useBattery is 94.6% of statements and 77.8% of branches. Two lines are not covered: the early return when the API is missing, and the dev only warning. Both have a comment saying why.

CI has not run here yet. Fork PRs need a maintainer to approve the run.

Two things I would like your opinion on:

• If getBattery() fails, the hook returns {isSupported: true, fetched: false}. That looks the same as still loading. An error field would fix it, but it adds to the public API. I left it out. I can add it if you want.
• isSupported is false on the server and true on the first client render. useNetworkState behaves the same way and takes an optional initialState. I did not add that here. I can if you want.

useBattery is still unchecked in #33. #1394 adds the same hook. I am happy to close this one if you prefer that.