Fix SIGABRT crash from background-location Info.plist misconfiguration

[billdmar]

The crash

The app aborted on launch with SIGABRT once "Always" location authorization had been granted (e.g. after testing the location prompt once). Crash report pinpointed it:

EXC_CRASH (SIGABRT) — CoreLocation NSException
  LocationManager.enableBackgroundUpdatesIfPermitted()  SafetyWatcherView.swift
  LocationManager.startTracking()
  SafetyWatcherView.body  (first layout)

Root cause

LocationManager sets allowsBackgroundLocationUpdates = true, but the built Info.plist was missing UIBackgroundModes = location. CoreLocation throws (→ abort()) when that property is set without the background mode declared.

Why it was missing: the project uses GENERATE_INFOPLIST_FILE = YES and declared INFOPLIST_KEY_UIBackgroundModes = location, but UIBackgroundModes is not one of the INFOPLIST_KEY_* names Xcode synthesizes into the generated plist — so it was silently dropped. (The location usage strings are synthesized keys, so those made it through, which is why the prompt appeared but the app then crashed.) This is pre-existing code from the background-location feature (#2) that had been compile-verified but never actually run.

Fix (two layers)

1. Make the feature work — add BuildConfig/Info.plist declaring UIBackgroundModes=[location] and point INFOPLIST_FILE at it (both Debug/Release). GENERATE_INFOPLIST_FILE=YES stays on and merges this file on top. It's kept outside the Party Watcher/ synchronized source folder on purpose — a plist inside it gets auto-added to Copy Bundle Resources and used as INFOPLIST_FILE, which produces a "Multiple commands produce Info.plist" build error.
2. Never crash again — harden enableBackgroundUpdatesIfPermitted() to only enable background updates if Bundle.main's UIBackgroundModes actually contains "location". Any future plist regression now degrades to foreground-only tracking instead of aborting.

Verification

• xcodebuild build → BUILD SUCCEEDED; built Info.plist now contains UIBackgroundModes = [location] (verified with PlistBuddy), usage strings retained.
• Installed on the Simulator, granted location-always (the exact condition that crashed before), launched → stays running, no new crash reports.
• Set a location → live map renders, chat/quick-actions/contacts all functional.

Investigation

Root cause and fix were confirmed by three parallel read-only investigators (crash-report analysis, plist-synthesis research, and a broader crash-risk audit) before any code changed.