Fix Garmin connect race, surface tool error content, add dev start scripts

- GarminConnection now flushes Profile's pending debounced autosave before
  connecting, avoiding a race where Connect fires with stale credentials.
- client.go reads res.Content before checking IsError so tool error
  messages actually include mcp-garmin's response text.
- seedsample: update kind lookups to match current taxonomy names
  ("Easy Run" -> "Easy", "Interval" -> "Intervals").
- Add backend/start.sh and frontend/start.sh dev launch scripts, and
  check in CLAUDE.md.

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

frontend/src/pages/Profile.tsx

@@ -88,6 +88,18 @@ export function Profile({ onSaved }: { onSaved?: (p: ProfileType) => void }) {
     saveTimeoutRef.current = setTimeout(() => persist(next), AUTO_SAVE_DELAY_MS);
   }
 
+  // GarminConnection's "Connect" button reads credentials from the backend's
+  // in-memory profile, which only updates once the debounced autosave above
+  // actually fires -- clicking Connect right after typing a password would
+  // otherwise race it and authenticate with stale (possibly empty) creds.
+  async function flushPendingSave() {
+    if (saveTimeoutRef.current) {
+      clearTimeout(saveTimeoutRef.current);
+      saveTimeoutRef.current = null;
+      if (profileRef.current) await persist(profileRef.current);
+    }
+  }
+
   if (!profile) {
     return (
       <div className="page">
@@ -132,7 +144,7 @@ export function Profile({ onSaved }: { onSaved?: (p: ProfileType) => void }) {
           value={profile.BackfillHorizonDays}
           onChange={(v) => set("BackfillHorizonDays", v)}
         />
-        <GarminConnection />
+        <GarminConnection onBeforeConnect={flushPendingSave} />
       </fieldset>
 
       {/* TODO: these settings currently have no explanation in the UI --