fix: collapsed-sidebar logo expands the menu; crypto nav icon matches the rest

Collapsed sidebar had no way to expand except the button at the bottom —
the logo at the top was purely decorative. It's now a real button in that
state (expanded state unchanged, still non-interactive there).

Swapped the Crypto nav icon from Bitcoin (a filled coin-badge glyph) to
Coins — same thin line-icon style as the rest of the sidebar (Layers,
Target, Flag, ...), where Bitcoin's brand-logo shape stood out. Bitcoin
stays as-is elsewhere (holdings table row fallback, Net Worth breakdown) —
appropriate there as a specific-asset placeholder, this was only about the
nav item.

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

frontend/src/lib/navigation.ts

@@ -1,8 +1,8 @@
 import {
   Activity,
   ArrowLeftRight,
-  Bitcoin,
   Calculator,
+  Coins,
   Flag,
   Layers,
   Lightbulb,
@@ -28,7 +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.crypto", to: "/crypto", icon: Coins },
   { labelKey: "nav.roi", to: "/roi", icon: Calculator },
   { labelKey: "nav.transactions", to: "/transactions", icon: ArrowLeftRight },
   { labelKey: "nav.accounts", to: "/accounts", icon: Layers },