feat: add a Crypto tab with live CoinGecko prices synced into Net Worth

Track individual coins (identity + quantity) in a dedicated tab, backed by
CoinGecko's Demo API (AURUM_COINGECKO_API_KEY). A holding is just another
Asset under the hood (asset_class=crypto) with its value written as a
regular AssetValuation — the existing Net Worth aggregation needs zero
changes to pick it up.

Prices never refresh in the background (no scheduler in this stack, same
reasoning as recurring transactions): a "Refresh prices" button forces a
sync, and GET /crypto/holdings lazily re-syncs at most once every 24h.
CoinGecko outages leave existing values untouched (error_key: "unreachable")
instead of failing the request. Backup/restore now carries CryptoHolding
rows too, so a restore doesn't silently strip a holding's coin identity.

Backend: 75/75 tests passing (9 new), including a real identity-map
staleness bug the tests caught in the raw-Core-upsert + ORM re-read pattern,
fixed via populate_existing=True.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01855PiW5Wd8n4aRW4n9kByY

frontend/src/lib/navigation.ts

@@ -1,6 +1,7 @@
 import {
   Activity,
   ArrowLeftRight,
+  Bitcoin,
   Calculator,
   Flag,
   Layers,
@@ -27,6 +28,7 @@ export interface NavItem {
 export const NAV_ITEMS: NavItem[] = [
   { labelKey: "nav.dashboard", to: "/", icon: LayoutDashboard },
   { labelKey: "nav.netWorth", to: "/net-worth", icon: TrendingUp },
+  { labelKey: "nav.crypto", to: "/crypto", icon: Bitcoin },
   { labelKey: "nav.roi", to: "/roi", icon: Calculator },
   { labelKey: "nav.transactions", to: "/transactions", icon: ArrowLeftRight },
   { labelKey: "nav.accounts", to: "/accounts", icon: Layers },