fix: mobile login flow

[peterchinman]

Fixes the login/auth flow for native mobile (Tauri) platforms.

• Router base path: Extracted hardcoded /app/... auth URLs into a shared routerBase constant so routing works on both web and native mobile.
• Mobile auth redirect: Unauthenticated mobile users go to /login instead of /signup.
• Login UI on mobile: Hides the 3D wireframe when the virtual keyboard is open, adjusts borders/layout accordingly, and uses onPointerDown instead of onClick on touch devices so buttons work before the keyboard dismisses.
• Chat input polish: Reduced right padding on touch devices, fixed send button sizing, changed base font size to text-sm, updated placeholder punctuation.
• Minor cleanup: Swapped some arcanum icons, fixed a malformed SVG attribute.

[github-actions]

Preview: https://peter-mobile-login-6sj7r7.preview.macro.com/app (945886c)

[claude]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.