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
