Warn at launch on development (dev branch) builds

[ps2]

Summary

• On each launch, after the status screen is constructed and displayed, present a blocking alert when the app was built from the LoopWorkspace dev branch, advising non-testers to switch to main.
• Branch is detected at runtime via BuildDetails.workspaceGitBranch == "dev" (the superproject branch captured into BuildDetails.plist at build time).
• Gated by a new devBranchWarningEnabled feature flag — on by default, disabled by adding DEV_BRANCH_WARNING_DISABLED to SWIFT_ACTIVE_COMPILATION_CONDITIONS (e.g. in LoopConfigOverride.xcconfig), matching the existing DIY flag pattern.
• The alert (.alert, non-dismissable except by a choice) offers I'm a tester (dismiss) and Switch to main (opens the LoopDocs branches page).
• Presented from LoopAppManager.launchHomeScreen(), alongside the existing AppExpirationAlerter launch-alert pattern, so it fires once per launch after onboarding.

Test plan

• On a dev build, the warning appears over the status screen on launch, and again on each subsequent launch.
• I'm a tester dismisses the alert; Switch to main opens the docs URL.
• No warning on main/release builds (workspaceGitBranch != "dev").
• Adding DEV_BRANCH_WARNING_DISABLED to the build's active compilation conditions suppresses the warning.