feat: Improve Bottle Split UX

- Add 'Split starten' button to bottle detail page
- Support flexible sample sizes (1-20cl) instead of fixed 5/10cl
- Preselect bottle when coming from bottle page
- Save sample sizes/shipping defaults to localStorage
- Update schema: sample_sizes JSONB array
- Update server actions and UI for dynamic sizes

supa_schema.sql

@@ -456,8 +456,7 @@ CREATE TABLE IF NOT EXISTS bottle_splits (
   total_volume INTEGER DEFAULT 70,  -- in cl
   host_share INTEGER DEFAULT 10,    -- what the host keeps, in cl
   price_bottle DECIMAL(10, 2) NOT NULL,
-  cost_glass_5cl DECIMAL(10, 2) DEFAULT 0.80,
-  cost_glass_10cl DECIMAL(10, 2) DEFAULT 1.50,
+  sample_sizes JSONB DEFAULT '[{"cl": 5, "glass_cost": 0.80}, {"cl": 10, "glass_cost": 1.50}]'::jsonb,
   shipping_options JSONB DEFAULT '[]'::jsonb,
   is_active BOOLEAN DEFAULT true,
   public_slug TEXT UNIQUE NOT NULL,
@@ -486,7 +485,7 @@ CREATE TABLE IF NOT EXISTS split_participants (
   id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
   split_id UUID REFERENCES bottle_splits(id) ON DELETE CASCADE NOT NULL,
   user_id UUID REFERENCES profiles(id) ON DELETE CASCADE NOT NULL,
-  amount_cl INTEGER NOT NULL CHECK (amount_cl IN (5, 10)),
+  amount_cl INTEGER NOT NULL CHECK (amount_cl > 0),
   shipping_method TEXT NOT NULL,
   total_cost DECIMAL(10, 2) NOT NULL,
   status TEXT DEFAULT 'PENDING' CHECK (status IN ('PENDING', 'APPROVED', 'PAID', 'SHIPPED', 'REJECTED', 'WAITLIST')),