feat: add mobile bottom navigation

[darwin67]

Summary

• replace the mobile hamburger trigger with a persistent, thumb-friendly bottom navigation
• expose the current organization or workspace destinations with clear active-page states
• keep organization and workspace switching in the accessible More drawer
• reserve mobile content space for the floating bar and support device safe-area insets

Verification

• mix precommit (428 ExUnit tests plus the Node navigation regression)
• mix assets.build
• browser verification at 390×844 for visibility, active state, content clearance, and More dialog behavior
• git diff --check

Routing

No routes changed. Organization and workspace destinations remain in their existing authenticated pipeline and LiveView sessions.