feat: multi-dimensional rest days + development routes architecture (v9d → v9e)

## Changes:

**Frontend:**
- Fix double icon in rest day list (removed icons from FOCUS_LABELS)
- Icon now shows once with proper styling

**Migration 011:**
- Remove UNIQUE constraint (profile_id, date) from rest_days
- Allow multiple rest day types per date
- Use case: Muscle recovery + Mental rest same day

**Architecture: Development Routes**
New document: `.claude/docs/functional/DEVELOPMENT_ROUTES.md`

6 Independent Development Routes:
- 💪 Kraft (Strength): Muscle, power, HIIT
- 🏃 Kondition (Conditioning): Cardio, endurance, VO2max
- 🧘 Mental: Stress, focus, competition readiness
- 🤸 Koordination (Coordination): Balance, agility, technique
- 🧘‍♂️ Mobilität (Mobility): Flexibility, ROM, fascia
- 🎯 Technik (Technique): Sport-specific skills

Each route has:
- Independent rest requirements
- Independent training plans
- Independent progress tracking
- Independent goals & habits

**Future (v9e):**
- Route-based weekly planning
- Multi-route conflict validation
- Auto-rest on poor recovery
- Route balance analysis (KI)

**Future (v9g):**
- Habits per route (route_habits table)
- Streak tracking per route
- Dashboard route-habits widget

**Backlog Updated:**
- v9d: Rest days ✅ (in testing)
- v9e: Development Routes & Weekly Planning (new)
- v9g: Habits per Route (extended)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/migrations/011_allow_multiple_rest_days_per_date.sql

@@ -0,0 +1,17 @@
+-- Migration 011: Allow Multiple Rest Days per Date
+-- v9d Phase 2a: Support for multi-dimensional rest (development routes)
+-- Date: 2026-03-22
+
+-- Remove UNIQUE constraint to allow multiple rest day types per date
+-- Use Case: Muscle recovery + Mental rest on same day
+-- Future: Development routes (Conditioning, Strength, Coordination, Mental, Mobility, Technique)
+
+ALTER TABLE rest_days
+DROP CONSTRAINT IF EXISTS unique_rest_day_per_profile;
+
+-- Add index for efficient queries (profile_id, date)
+CREATE INDEX IF NOT EXISTS idx_rest_days_profile_date_multi
+ON rest_days(profile_id, date DESC);
+
+-- Comment for documentation
+COMMENT ON TABLE rest_days IS 'v9d Phase 2a: Multi-dimensional rest days - multiple entries per date allowed for different development routes (muscle, cardio, mental, coordination, technique)';

frontend/src/pages/RestDaysPage.jsx

@@ -62,11 +62,11 @@ const PRESETS = [
 ]
 
 const FOCUS_LABELS = {
-  muscle_recovery: '💪 Muskelregeneration',
-  cardio_recovery: '🏃 Cardio-Erholung',
-  mental_rest: '🧘 Mentale Erholung',
-  deload: '📉 Deload',
-  injury: '🩹 Verletzungspause',
+  muscle_recovery: 'Muskelregeneration',
+  cardio_recovery: 'Cardio-Erholung',
+  mental_rest: 'Mentale Erholung',
+  deload: 'Deload',
+  injury: 'Verletzungspause',
 }
 
 export default function RestDaysPage() {
@@ -434,7 +434,8 @@ export default function RestDaysPage() {
                     color: preset?.color || '#888',
                     marginBottom: day.note ? 8 : 0,
                   }}>
-                    {preset?.icon || '📅'} {FOCUS_LABELS[focus] || focus}
+                    <span style={{ fontSize: 14 }}>{preset?.icon || '📅'}</span>
+                    {FOCUS_LABELS[focus] || focus}
                   </div>
 
                   {/* Note */}