feat: crypto portfolio analytics (history chart, allocation, P&L, cost basis)

Adds the analytics half of the CoinMarketCap reference: a value-over-time
chart (GET /crypto/history, forward-filled from AssetValuation like Net
Worth's own series but scoped to crypto assets), an allocation breakdown by
coin, and stat tiles for all-time profit, total invested (cost basis), and
best/worst performer.

Deliberately no "24h" range — history is only as dense as the sync cadence
(button or once/day), so a 24h chart would be one or two points, not a
smooth intraday line. Cost basis / all-time profit / best-worst performer
aren't separate endpoints: they're trivial to derive client-side from
fields already in GET /crypto/holdings, so no redundant backend logic.

Allocation card follows the dataviz skill's categorical rule: top 7 coins
by value get a fixed-order color from the app's already-validated 8-slot
ramp, an 8th+ folds into "Other" rather than generating a new hue.

82/82 backend tests passing (3 new for /crypto/history). Verified live
against the real CoinGecko API — and caught a real holding the user had
already added directly through the UI between messages, which the smoke
test correctly left untouched while cleaning up only its own test data.

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

frontend/src/api/crypto.ts

@@ -1,7 +1,9 @@
 import { api } from "@/api/client";
 import type {
+  CryptoHistoryResponse,
   CryptoHolding,
   CryptoHoldingCreateInput,
+  CryptoRange,
   CryptoSearchResult,
   CryptoSyncResult,
   CryptoTransaction,
@@ -45,3 +47,7 @@ export function deleteCryptoHolding(assetId: number) {
 export function searchCryptoCoins(query: string) {
   return api.get<CryptoSearchResult[]>(`/crypto/search?q=${encodeURIComponent(query)}`);
 }
+
+export function fetchCryptoHistory(range: CryptoRange) {
+  return api.get<CryptoHistoryResponse>(`/crypto/history?range=${range}`);
+}