feat(startup): prepare the main app session PR 3

[MohamadJaara]

Goal

Prepare the current user's database before the main app graph and navigation start.

Changes

• make main startup wait for the shared preparation gate
• build the session graph from the prepared user scope
• show the migration screen only when preparation is long-running or fails
• support retry, update, and support actions from startup
• cover startup and session graph behavior with focused tests

Stack

• Epic branch: mo/epic/async-db-migration
• PR 1
• PR 2
• PR 3 of 7

Merge the stack bottom-up into the epic branch.