fix(mobile): tiebreak priority sort by newest first (port #2787)

[Gilbert09]

Summary

Ports desktop PR #2787 (fix(inbox): tiebreak priority sort by newest first) to the React Native / Expo mobile app.

Priority is a coarse 5-bucket rank (P0–P4); without a secondary sort, reports within the same priority tier came back in arbitrary order on mobile. This appends a -created_at (newest first) tiebreak to the signal report list ordering param when sorting by priority, so the ordering string becomes e.g. status,-is_suggested_reviewer,-priority,-created_at. The existing status and -is_suggested_reviewer prefixes are unchanged, and non-priority sort fields are unaffected.

Changes

• apps/mobile/src/features/inbox/utils.ts — add the -created_at tiebreak in buildSignalReportListOrdering for the priority field only.
• apps/mobile/src/features/inbox/utils.test.ts — parameterized tests across all ordering fields and both directions, asserting the tiebreak is present for priority and absent for other fields.

Testing

• vitest run src/features/inbox/utils.test.ts — 37 passing.
• Lint clean on changed files; no new typecheck errors introduced.

Scope is limited to apps/mobile; no changes to apps/code or packages/*.

[github-actions]

React Doctor found no issues in the changed files. 🎉

_{Reviewed by React Doctor for commit 81fd716.}

[greptile-apps]

_{Reviews (1): Last reviewed commit: "fix(mobile): tiebreak priority sort by n..." | Re-trigger Greptile}