fix(sync): suppress desktop 'zero peers' warning in Sync Settings; show push-only note

[TimeToBuildBob]

Summary

Closes #291 (re-scoped on 2026-09-18 by Erik).

On Android, pull is a no-op by construction — the SAF mirror is outbound-only so
pull_all() always finds zero peers. This causes aw-server-rust#687's
"zero peers in a configured sync dir is usually a layout or setup problem" warning
to appear in Sync Settings on every run, misleading users into thinking sync is broken.

Per the re-scope: Android stays push-only until sync v2 (JSONL+zstd
read-from-staging). Until then Sync Settings should:

1. Not show the desktop-only "zero peers" warning
2. Show "Push-only on this device (pull arrives with sync v2)" so users understand
   why pull counts are always 0

Changes

SyncSettingsActivity.kt

• formatSyncStatus and formatSyncDetail gain an isPushOnlyDevice: Boolean = false
  parameter (default false — all non-Android call sites are unchanged)
• When isPushOnlyDevice = true:
  • warnings containing "zero peers" are filtered from the detail line
  • "Push-only on this device (pull arrives with sync v2)" is appended to every
    last-sync status display (including the "Last sync: never" initial state)
• updateLastSyncStatus() now passes isPushOnlyDevice = true

SyncSettingsActivityTest.kt — 3 new unit tests:

• formatSyncStatus_pushOnlyDevice_suppressesZeroPeersWarning — zero-peers warning filtered, push-only note appended
• formatSyncStatus_pushOnlyDevice_preservesOtherWarnings — other warnings (e.g. "push aborted after pull failure") still surface
• formatSyncStatus_pushOnlyDevice_appendsNoteEvenWhenNeverSynced — note appears even before the first sync

Item 2: JNI sync dir root verification

Erik's re-scope also asks to verify what root getSyncDir() returns at the JNI boundary
(…/files/sync vs …/files/sync/poco_f8_ultra). That diagnostic is tracked as a
follow-up: running the app on device and reading Log.d "getSyncDir()" from logcat before
the v2 reader is designed. Not included in this UI-only PR.

Test plan

• Build and run on device; open Sync Settings after a completed sync
• Confirm "zero peers in a configured sync dir" no longer appears in the status
• Confirm "Push-only on this device (pull arrives with sync v2)" appears below the counts
• ./gradlew :mobile:testDebugUnitTest passes (unit tests are pure JVM, no emulator needed)

[greptile-apps]

Confidence Score: 5/5

The PR appears safe to merge, with the new presentation behavior consistently applied across Sync Settings refresh paths.

Summary

This PR adjusts Android Sync Settings to accurately describe the current outbound-only synchronization behavior.

• Suppresses the desktop-specific “zero peers” warning on push-only devices.
• Adds a push-only explanatory note to completed, legacy, and never-synced status displays.
• Preserves other synchronization warnings.
• Adds focused unit coverage for warning filtering, warning preservation, and the initial state.

_{Reviews (1) · Last reviewed commit: "fix(sync): suppress desktop 'zero peers'..."}

[TimeToBuildBob]

The E2E failure on the Test E2E (29) job (NativeWindowInsetsTest.syncToggleReceivesRealTap) is unrelated to this PR.

The PR only touches formatSyncStatus, formatSyncDetail, and their unit tests — zero lines of toggle logic (setSyncEnabled, setOnCheckedChangeListener, switch_sync_enabled) changed. The flaky test is a known E2E fragility hardened in #287 (merged 2026-09-17); the monitoring task is still collecting consecutive-clean-run evidence on master.

I don't have write access to re-trigger CI. Requesting a re-run of the failed job — the unit tests all pass and Greptile gave it 5/5.