fix: clarify iOS location and motion permission copy

[janicduplessis]

Description

The legacy always-location and motion usage descriptions used generic generated wording. Both now name Trailhead and explain the benefit: keeping a hike recording active in the background and improving tracking with motion activity.

Only NSLocationAlwaysUsageDescription and NSMotionUsageDescription changed. No prebuild or other native configuration change was made.

Closes #9

Test plan

• Validated the source plist with plutil -lint.
• Confirmed the iOS native fingerprint missed because ios changed, triggering an Xcode rebuild.
• Extracted both values from the newly installed Trailhead.app/Info.plist and confirmed they match the source.
• Reset location permission and repeated the Record flow on the same Stim-owned iOS simulator.

Stim launch summary:

fingerprint 497d13.. miss -- 1 source changed: ios
build       compiling Debug with xcodebuild
OK: com.appandflow.trailhead on stim-issue-9-ios-permission-copy (261D..), Metro port 8087

Before

Source plist:

NSLocationAlwaysUsageDescription = Allow $(PRODUCT_NAME) to access your location
NSMotionUsageDescription = Allow $(PRODUCT_NAME) to detect your current motion activity

Installed product:

Allow Trailhead to access your location
Allow Trailhead to detect your current motion activity

After

Source and installed product:

NSLocationAlwaysUsageDescription = Trailhead uses your location in the background to keep recording your hike when the app is not open.
NSMotionUsageDescription = Trailhead uses motion activity to improve hike tracking while you record.

The prompt screenshots are contextual rather than a visual before/after of either changed string. Apple documents that current iOS uses the unchanged NSLocationAlwaysAndWhenInUseUsageDescription; NSLocationAlwaysUsageDescription is shown only on iOS versions before 11. The app has no motion-permission request in its UI flow, and the simulator does not provide motion activity, so showing NSMotionUsageDescription would require an out-of-scope behavior change.