Merge backfill into Sync now, replace Full backfill with a destructive Reset all

Sync now now runs Backfill (resumes from the watermark, so a widened history
horizon is picked up automatically) then IncrementalSync then detail-fill, in
one click. "Full backfill" is gone -- in its place, "Reset all" (styled as a
destructive action, gated by a confirm dialog) wipes every synced activity
and its laps/kind assignments and rewinds the backfill watermark, so the next
sync performs a genuinely fresh pull instead of trying to patch up existing
rows with newer schema fields.

frontend/src/components/GarminConnection.tsx

@@ -68,11 +68,27 @@ export function GarminConnection() {
     }
   }
 
-  async function sync(kind: "run" | "backfill") {
+  async function sync() {
     setBusy(true);
     setError(null);
     try {
-      await (kind === "run" ? api.syncRun() : api.syncBackfill());
+      await api.syncRun();
+      refreshStatus();
+    } catch (e) {
+      setError(String(e));
+    } finally {
+      setBusy(false);
+    }
+  }
+
+  async function resetAll() {
+    if (!window.confirm("This deletes every synced activity, lap, and kind assignment, then starts a fresh pull from Garmin next sync. This cannot be undone. Continue?")) {
+      return;
+    }
+    setBusy(true);
+    setError(null);
+    try {
+      await api.resetSync();
       refreshStatus();
     } catch (e) {
       setError(String(e));
@@ -102,11 +118,11 @@ export function GarminConnection() {
 
         {status === "authenticated" && (
           <>
-            <button disabled={busy} onClick={() => sync("run")}>
+            <button disabled={busy} onClick={sync}>
               Sync now
             </button>
-            <button disabled={busy} onClick={() => sync("backfill")}>
-              Full backfill
+            <button className="button-danger" disabled={busy} onClick={resetAll}>
+              Reset all
             </button>
             {syncStatus?.in_progress && (
               <span className="status-label">{syncProgressLabel(syncStatus)}</span>