feat(admin): add proper EmDash logo branding to admin UI (#200)

Replace placeholder text branding ("— EmDash") with actual logo SVGs
from the brand assets. Adds Logo.tsx with LogoIcon (icon mark) and
LogoLockup (icon + wordmark) components.

- Sidebar: gradient icon mark replaces em dash text character
- Login, Signup, Setup: full lockup SVG with currentColor wordmark
- Welcome modal: logo icon replaces Sparkle placeholder
- Favicon: real gradient icon SVG replaces emoji

.changeset/admin-logo-branding.md

@@ -0,0 +1,6 @@
+---
+"@emdash-cms/admin": patch
+"emdash": patch
+---
+
+Replace placeholder text branding with proper EmDash logo SVGs in admin UI