fix: show asset suggestions on empty Add Asset query [PERA-4159]

[wjbeau]

Summary

• Matches the Android behavior on the Add Asset / Add Collectible screens: when the search field is empty, fetch and show the backend's default suggestion list instead of leaving the screen blank.
• Adds a showOnEmptyQuery opt-in to useGlobalSearch so the remote asset query runs (and surfaces loading) without typed input. Local sections (accounts/contacts/owned assets) remain empty until the user types.
• Omits the q parameter from /v1/assets/search/ when it's blank so the API returns suggestions (Android sends q as null, which Retrofit drops; RN was sending q='').

Test plan

• Open Add Asset on a non-watch account — assets list should populate immediately (not blank).
• Open Add Collectible — collectibles list should populate immediately.
• Type a query — results should filter as before; clear the field — suggestions return.
• Compare side-by-side with Android prod build to confirm parity.
• pnpm build passes (tsc).
• useGlobalSearch and assets endpoint test suites pass with new coverage.

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