[ORG] Wire up Join Organization button (JoinOrganizationBottomSheet)

[CRamsan]

Is your feature request related to a problem? Please describe.
On the My Organizations screen, the "+ Join Organization" button is currently a no-op stub — it just shows a "Coming soon!" snackbar (MyOrganizationsViewModel.navigateToJoinOrganization()). Users cannot actually join an organization from this entry point.

Describe the solution you'd like
Implement the Join Organization flow (a JoinOrganizationBottomSheet or equivalent screen) that lets a user enter an invite code or search for an organization and join it, and wire it up to MyOrganizationsScreen's "+ Join Organization" button in place of the current stub.

Describe alternatives you've considered
N/A

Additional context

• Mock already exists: edifikana/mockups/join-organization.html
• This was previously tracked under [AUTH] Invitation accept flow — screens, ViewModel, deep link (Phase 1) #391, but that issue is actually about the (closed) Invitation Accept flow, not Join Organization — edifikana/index.md's "Join Organization" row was mislabeled with [AUTH] Invitation accept flow — screens, ViewModel, deep link (Phase 1) #391 and will be corrected to point here.
• Related: [ORG] My Organizations screen + Org Detail + Leave/Transfer ownership (Phase 1) #392, [FEATURE] My Organizations screen #541