Add simplified device backup flow

[MiSikora]

Task/Issue URL: https://app.asana.com/1/137249556945/project/1216103556496795/task/1216422585541210?focus=true
Tech Design URL (if applicable): https://app.asana.com/1/137249556945/project/1216103556496795/task/1216509582049467?focus=true

Description

Adds the "back up this device" flow to the simplified sync UI. When a signed-out user turns on the Sync this device toggle on the new sync settings screen, they now go through a dedicated intro screen and can create a sync account to back this device up.

Added:

• Sync This Device intro screen (SyncThisDeviceActivity) with two CTAs — "Sync With Another Device" and "Sync This Device Only".
• Account creation ("Sync This Device Only") — creates the sync account and reports into the sync-setup wide-event funnel.
• Toggle → flow wiring in the v2 SyncActivity via SyncThisDeviceContract (ActivityResultContract), including device authentication, device-unsupported and passcode-enrollment guards.

Not implemented here (follow-up PRs):

• The recovery code screen is a placeholder (RecoveryCodeActivity). A successful backup routes to it, but showing/saving the recovery code (and the PDF) will come in its own PR — I'd rather isolate it.
• The Sync With Another Device button on the intro screen isn't wired to the pairing flow yet (it currently returns to settings).

Steps to test this PR

Back up this device (simplified sync)

• Go to the "Sync Dev Settings".
• Tap "Launch Sync Settings V2".
• Toggle "Sync & Backup this device".
• Confirm the intro screen shows both CTAs.
• Tap the X on the intro screen.
• Confirm that it returns to settings with the toggle unchecked.
• Toggle "Sync & Backup this device".
• Tap "Sync This Device Only".
• Complete device authentication.
• Confirm the account is created (device shows as backed up / signed in).

UI changes

Intro page	Syncing state

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Note

Medium Risk
Touches sync account creation, device authentication, and signup analytics on a new user-facing path; recovery and several v2 settings commands are still unimplemented.

Overview
Adds the simplified “back up this device” path on sync settings v2: turning on Sync & Backup this device runs device-auth / passcode enrollment checks, then opens a dedicated intro via SyncThisDeviceContract.

Sync This Device intro (SyncThisDeviceActivity) offers Sync With Another Device (returns to settings; pairing not wired yet) and Sync This Device Only, which creates a sync account through SyncAccountRepository, shows a syncing state, and emits sync-setup wide events and signup pixels. Cancel or dismiss resets the settings toggle via new SyncThisDeviceCanceled handling in v2 SyncActivity (legacy sync activity ignores that command).

Successful backup refreshes the device list and navigates to a placeholder RecoveryCodeActivity. Most other v2 SyncActivity commands remain TODO stubs. Manifest registers the new activities; strings and layout support the intro screen.

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[MiSikora]

• Add simplified sync edit device UI #9221
• Add simplified device backup flow #9210 👈 (View in Graphite)
• develop

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