fix(wear): open-on-phone button + reactive auth refresh (#44), duplicate-front message (#45)

[SiteRelEnby]

Fixes #44 and #45.

#44 — "Open Sheaf on phone" screen

Button. The watch's "Open Sheaf on your phone to sign in" screen now has an Open on phone chip. It uses RemoteActivityHelper (system-mediated remote start, so it isn't blocked by the phone's background-activity-launch limits) with a sheaf://open deep link the phone app now registers. The phone's MainActivity just comes to the foreground; the URL carries no redeem params, so captureRedemptionDeepLink ignores it. Toasts "Couldn't reach your phone" if the start fails.

Reactive refresh. When the phone pushes a new token, that screen now updates itself instead of needing a leave-and-return. The auth-state flow was a per-WearAuthManager-instance field, and the data-layer service, the activity, tiles and complications each build their own instance over the same encrypted store. So the service applying a phone-pushed credential updated only its flow, and the login screen (the activity's instance) never saw it. Hoisted the flow into a process-wide singleton, so any instance's saveCredentials/clearCredentials updates the exact flow the UI collects. The existing signal-counter listener stays as a belt-and-suspenders for any write that bypasses this class.

#45 — "switch failed" on a duplicate combination

Creating a front whose exact member set already has an open front returns 409 from the server (fronts.py), which the watch surfaced as a bare "Switch failed" / "error 409". Added a SwitchOutcome (SWITCHED / ALREADY_FRONTING / FAILED); switchFront maps 409 to ALREADY_FRONTING (not queued, not an error, since the state the user wanted already holds) and the callers word it: " already fronting" on the quick-switch tile toast, "Already fronting" on the switch screen. Other permanent 4xx still read as a failure.

Notes

• New dependency: androidx.wear:wear-remote-interactions:1.0.0 (wear module).
• New phone deep link: sheaf://open (host-only, BROWSABLE) on MainActivity.

Testing

:app:assemblePlayRelease :wear:assembleRelease :wear:testPlayReleaseUnitTest green. Device checklist before merge:

• Open-on-phone chip launches the phone app from the watch sign-in screen
• Watch sign-in screen advances to the menu on a phone token push, without leaving and re-entering
• Switching to a member set that's already fronting shows "already fronting", not "switch failed"