Fix/improve deep link race conditions

[StylianosGakis]

This is an attempt to fix the race conditions around deep links by making sure reconcilation happens first, and then if a deep link is pending, it is handled appropriately, meaning taking over the entire backstack if required.

Need to test a bit more before merge

AI description of the reason the new timing corodingation which works well compared to letting the coroutines race each other:

1. The collector now waits. It calls externalDeepLinkHandler.handle(uri), which is suspend and whose first line is readySignal.first { it }. It cannot touch the back stack until isReady == true. (Before: synchronous openUri, no wait.)
2. navigateToExternalDeepLink lands alone in both auth states: logged out → pendingDeepLink = key (consumed later by setLoggedIn, which lands it alone); logged in → reseed(listOf(key)) → stack becomes exactly [destination]. Neither branch ever appends onto an existing root. (Before: logged-in branch did remove + add = append.)
3. reconcile() sets the root, then flips the gate: determineStartScene() settles the root first, and only then isReadyState.value = true. So the moment the gate opens, the root is already final. (Before: same internal order, but nothing downstream waited for it.)
4. The gate enforces the ordering the launch order only hinted at. MainActivity still launches reconcile() then setContent, but now even if the collector starts first and the auth/member-id flows suspend, handle() parks on readySignal.first { it } and yields back — it physically cannot run before reconcile finishes.

So the deciding factor from before — did the auth/member-id flows emit synchronously or suspend? — no longer changes the result:

- Flows suspend → handle() blocks on the gate until reconcile completes → reads the settled state → lands alone.
- Flows cached → reconcile completes first anyway → handle() reads the same settled state → lands alone.

Both timings now converge on one outcome: [destination] alone, Home never beneath it, system Back exits to Slack. The two random outcomes collapse into one. That’s
the determinism — it comes from the gate (point 1) removing the timing dependence, and reseed (point 2) removing the “append onto Home” outcome even in the logged-in
branch.