feat: advanced search with collapsible settings and author filter

[barrydeen]

Summary

• Restores PR feat: advanced search with author filter #278 which was orphaned from main when PR fix: replace relay self-reconnect with external sweep and improve backoff #279 was merged off an older base
• Moves search relay selector into a collapsible "Advanced search" section
• Adds author filter for note search — search for people inline and select one to filter notes by author
• Adds search icon on profile screen top bar that navigates to search pre-filtered by that user

What was lost

PR #278 was merged to main on 2026-03-19 but PR #279 was based on PR #277, causing the merge to bypass #278's changes entirely. The merge commit 03e3be27 existed in git but was unreachable from main.

Files restored

• SearchScreen.kt — collapsible Advanced search section, AuthorFilter composable
• SearchViewModel.kt — authorFilter state, searchAuthors(), prepareAuthorSearch()
• UserProfileScreen.kt — search icon that pre-fills author filter
• Navigation.kt — onSearchAuthor callback wiring