Decompose IOURequestStepParticipants and extract shared start-page hooks

[Julesssss]

Sub-issue of #81849, coming from here.

Comment related to this task.

Task

IOURequestStepParticipants has 17 Onyx subscriptions but only 1 is used for rendering; the other 16 are callback-only. This causes unnecessary re-renders in the Create Expense flow.

• Split into a router with variants per flow
• Move callback-only subscriptions behind refs
• Extract duplicated tab-reset logic shared between IOURequestStartPage and DistanceRequestStartPage into shared hooks

[melvin-bot]

@OlimpiaZurek Huh... This is 4 days overdue. Who can take care of this?

[Julesssss]

Related PRs in progress