fitness: add per-exercise metrics, cardio support, and stats page

- Add metrics system (weight/reps/rpe/distance/duration) per exercise type
  so cardio exercises show distance+duration instead of weight+reps
- Add 8 new cardio exercises: swimming, hiking, rowing outdoor, cycling
  outdoor, elliptical, stair climber, jump rope, walking
- Add bilateral flag to dumbbell exercises for accurate tonnage calculation
- Make SetTable, SessionCard, history detail, template editor, and exercise
  stats API all render/compute dynamically based on exercise metrics
- Rename Profile to Stats with lifetime cards: workouts, tonnage, cardio km
- Move route /fitness/profile -> /fitness/stats, API /stats/profile -> /stats/overview

src/models/WorkoutSession.ts

@@ -1,9 +1,11 @@
 import mongoose from 'mongoose';
 
 export interface ICompletedSet {
-  reps: number;
+  reps?: number;
   weight?: number;
   rpe?: number; // Rate of Perceived Exertion (1-10)
+  distance?: number; // km
+  duration?: number; // minutes
   completed: boolean;
   notes?: string;
 }
@@ -34,7 +36,6 @@ export interface IWorkoutSession {
 const CompletedSetSchema = new mongoose.Schema({
   reps: {
     type: Number,
-    required: true,
     min: 0,
     max: 1000
   },
@@ -48,6 +49,16 @@ const CompletedSetSchema = new mongoose.Schema({
     min: 1,
     max: 10
   },
+  distance: {
+    type: Number,
+    min: 0,
+    max: 1000 // km
+  },
+  duration: {
+    type: Number,
+    min: 0,
+    max: 6000 // minutes
+  },
   completed: {
     type: Boolean,
     default: false

src/models/WorkoutTemplate.ts

@@ -1,9 +1,11 @@
 import mongoose from 'mongoose';
 
 export interface ISet {
-  reps: number;
+  reps?: number;
   weight?: number;
   rpe?: number; // Rate of Perceived Exertion (1-10)
+  distance?: number; // km
+  duration?: number; // minutes
 }
 
 export interface IExercise {
@@ -27,8 +29,7 @@ export interface IWorkoutTemplate {
 const SetSchema = new mongoose.Schema({
   reps: {
     type: Number,
-    required: true,
-    min: 1,
+    min: 0,
     max: 1000
   },
   weight: {
@@ -40,6 +41,16 @@ const SetSchema = new mongoose.Schema({
     type: Number,
     min: 1,
     max: 10
+  },
+  distance: {
+    type: Number,
+    min: 0,
+    max: 1000 // km
+  },
+  duration: {
+    type: Number,
+    min: 0,
+    max: 6000 // minutes
   }
 });