Investment portfolio charts, search fix, and holding creation fixes

- Add four portfolio charts: allocation donut by holding, allocation
  donut by asset type, cost basis vs current value bar, return % bar
- Fix asset search to use yf.Search() full-text instead of ticker-only
  lookup — name searches like "vanguard ftse all world" now work
- Fix holding creation double-quantity bug: holdings now created with
  quantity=0 so buy transaction is sole source of quantity/cost basis
- Add per-share / total price toggle in Add Holding modal with live
  calculated equivalent shown as you type
- Add ErrorBoundary in AppShell so render errors show a message instead
  of a blank page
- Fix donut charts using || instead of ?? when falling back from
  current_value to cost_basis_total (0 was not falling through ??)
- Allow HoldingCreate.quantity >= 0 (was gt=0) to support zero-init
- Fix error display for Pydantic v2 array-of-objects validation errors

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

backend/app/schemas/investment.py

@@ -36,7 +36,7 @@ class AssetPricePoint(BaseModel):
 class HoldingCreate(BaseModel):
     account_id: uuid.UUID
     asset_id: uuid.UUID
-    quantity: Decimal = Field(..., gt=0)
+    quantity: Decimal = Field(..., ge=0)
     avg_cost_basis: Decimal = Field(..., ge=0)
     currency: str = Field(default="GBP", min_length=3, max_length=10)