test(sync): cover FullSync's phase transitions end-to-end

TestFillPendingDetails_ReportsPhaseAwareProgressThroughActivitiesAndWorkoutsPhases
only ever called FillPendingDetails directly, never exercising
FullSync's own setProgress(PhaseDiscovering, ...) / deferred
setProgress(PhaseIdle, ...) wiring around backfillCore/
incrementalSyncCore -- what handleSyncRun actually runs in production
and what SyncModal's "Discovering activities..." spinner depends on.

Add a mock.Client.Delay field (slept, ctx-cancellable, at the start of
GetActivities) so a test can give the discovering phase real
wall-clock duration, then add
TestFullSync_ReportsPhaseTransitionsThroughDiscoveringToIdle, which
runs FullSync in a goroutine and polls Progress() to confirm it
observes PhaseDiscovering mid-flight and PhaseIdle after completion.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/internal/garmin/mock/mock.go

@@ -4,6 +4,7 @@ package mock
 
 import (
 	"context"
+	"time"
 
 	"geniusrun/backend/internal/garmin"
 )
@@ -20,7 +21,13 @@ type Client struct {
 	// all-calls-fail Err field below -- so a test can simulate one
 	// activity's workout fetch failing while others in the same batch
 	// succeed.
-	WorkoutErrByID     map[int64]error
+	WorkoutErrByID map[int64]error
+	// Delay, if set, is slept (ctx-cancellable) at the start of every
+	// GetActivities call -- lets a test give sync.Service's discovering
+	// phase (backfillCore/incrementalSyncCore) real wall-clock duration, so
+	// a concurrent goroutine can observe Progress() mid-flight instead of
+	// the call returning instantly.
+	Delay              time.Duration
 	Err                error // if set, every call returns this error
 	authResultCursor   int
 	ClosedCalled       bool
@@ -58,6 +65,13 @@ func (c *Client) CompleteMFA(ctx context.Context, code string) (garmin.AuthResul
 
 func (c *Client) GetActivities(ctx context.Context, startDate, endDate string, limit int) ([]garmin.Activity, error) {
 	c.GetActivitiesCalls++
+	if c.Delay > 0 {
+		select {
+		case <-ctx.Done():
+			return nil, ctx.Err()
+		case <-time.After(c.Delay):
+		}
+	}
 	if c.Err != nil {
 		return nil, c.Err
 	}