feat(ui): 3-tab UI per spec — admin dashboard + date filter, personal dashboard, training w/ difficulty, settings profile

Tab 1 Admin overview (/): aggregate stats + DATE FILTER (?from&to), hardest personas,
admin quick actions (Users / + Add product).
Tab 2 My dashboard (/my/board): per-user win/lose/session summary (works for admin too).
Tab 3 Training (/training): product list w/ status + persona count; admins see all groups
(draft->analyze/edit), trainees see ready->personas; persona list shows DIFFICULTY (1-5
stars); admins get a Manage-personas button.
Settings (/settings): profile (name/email) via new PATCH /api/auth/profile + change
password + language.
Backend: analytics date filter; profile endpoint; /api/me/board + /api/chat/sessions
opened to any authed user.
Rebuilt frontend/dist.

frontend/src/style.css

@@ -90,6 +90,8 @@ button { min-height: 44px; transition: transform .15s ease, box-shadow .2s ease,
 button:not(:disabled):hover { box-shadow: 0 4px 12px rgba(20,24,40,.1); }
 button:not(:disabled):active { transform: scale(.97); }
 button.primary:not(:disabled):hover { box-shadow: 0 6px 18px rgba(79,70,229,.35); }
+button.soft { background: #f1f3f9; border-color: transparent; color: var(--ink); }
+button.soft:not(:disabled):hover { background: #e6eaf3; }
 
 input, select, textarea {
   min-height: 44px;