fix(ios): stop empty-state flash during data browser reload

[datlechin]

Summary

Two related iOS data browser bugs reported via TestFlight (iPhone 17 Pro / iOS 26.4.2) and reproduced locally:

1. Clear the search field on a PostgreSQL table → "No Data" empty state shows briefly instead of returning to the row list.
2. Any reload that follows a sort, filter, or page-size change flashes "No Data" between clearing the previous rows and the new rows streaming in.

Root cause

DataBrowserViewModel.load() only set isLoading = true when legacyRows.isEmpty || isInitial. The downstream loadPage() then unconditionally calls legacyRows.removeAll() before streaming begins. For any reload that started with rows on screen (i.e., sort, filter, search, clear-search), legacyRows became empty mid-flight while both loading flags read false. The view's if rows.isEmpty branch fired in that window and rendered the "No Data" empty state, which then got replaced when rows arrived.

navigatePage() set isPageLoading manually around its load() call, but the other entry points (applySort, applyFilters, clearFilters, applySearch, clearSearch) did not.

Fix

• load() now owns both loading flags. If isInitial || legacyRows.isEmpty it sets isLoading = true (full-screen spinner); otherwise it sets isPageLoading = true (in-list overlay that keeps the existing rows visible until they get replaced). Both flags reset on every exit path.
• navigatePage() no longer touches isPageLoading since load() handles it.
• DataBrowserView.content gets one new branch: rows.isEmpty && isPageLoading shows a ProgressView instead of falling through to the empty state.
• Bottom toolbar visibility includes !viewModel.isPageLoading so it stops flickering when an interaction clears rows.

Test plan

• Search on a PostgreSQL table, clear the search field → row list returns directly with no "No Data" flash
• Apply a filter, then clear it → list reloads without empty-state flash
• Change sort column, change page size → in-list overlay shows, no empty state
• Open a genuinely empty table → "No Data" with Insert Row action still shows
• Search with zero matches → ContentUnavailableView.search still shows
• DataBrowserViewModelTests/clearSearchReplacesRowsCleanly passes