Supersedes #1300 with a deeper UX refactor matching TablePlus's pattern. Solves #1296 by adding a proper schema switcher instead of a tree of all schemas.

What changed vs the original PR #1300

The original PR added "show all schemas as collapsible sections" in the sidebar — a tree-of-schemas model. Problems:

• Tree gets unusable at scale (Supabase has 8+ schemas — auth, storage, realtime, extensions, graphql, pgsodium, public, vault).
• Conflates two concepts: "browsing schemas" vs "switching active schema".
• The cross-schema click handling went through switchSchema → connection-wide state mutation that thrashed other windows.
• TableInfo identity regression for non-grouped users (schema: table.schema ?? resolvedSchema over-fallback in PluginDriverAdapter).

This branch instead implements the TablePlus pattern:

• Sidebar shows ONE active schema at a time (flat by capability: Tables / Views / Procedures / Functions sections, like today's flat mode).
• SchemaPickerFooter at the bottom of the sidebar — native NSPopUpButton(pullsDown: true) via NSViewRepresentable. Opens upward as a real NSMenu with proper HIG bezel/chevron/hover.
• DatabaseSwitcher popover lost its schema mode (used to have a segmented control toggling Databases/Schemas). Databases-only now.
• System schemas hidden by default, "Show System Schemas" toggle menu item in the picker.
• Footer hidden when the database has ≤ 1 schema, or for drivers without schema support (MySQL, SQLite, Redis, MongoDB).

Architectural fixes (the bugs PR #1300 had)

1. Cross-schema SELECT now qualifies properly. Clicking `auth.audit_log_entries` from any schema-aware sidebar produces `SELECT * FROM "auth"."audit_log_entries" LIMIT 1000` instead of the connection's currentSchema-qualified or unqualified variants. Three call paths fixed:

   • Double-click via `MainSplitViewController.onDoubleClick`
   • Single-click via `TableSelectionAction.navigate(table: TableInfo)` (previously decomposed as `(tableName, isView)` losing schema)
   • New-window via `EditorTabPayload` + `SessionStateFactory` (was passing schemaName as metadata only, not to the SQL builder)

2. Canonical `openTableTab(_ table: TableInfo)` entry. The String-based form is kept as the internal primitive for FK navigation / quick switcher / create-table where the caller genuinely doesn't have a TableInfo. Stops the parameter-explosion smell.

3. `TableInfo` identity preserved for legacy callers. `PluginDriverAdapter.fetchTables()` doesn't apply schema fallback; only `fetchTables(schema:)` does. Selection sets, tab restoration via `TabPersistenceService`, and ER Diagram layouts no longer churn.

4. Schema fan-out serialized. `SchemaService` no longer issues `3 * N` concurrent queries (one per schema per kind) on the shared driver connection. Doesn't apply anymore since the grouped path is gone, but the underlying `fetchTablesAcrossSchemas` was serialized first then removed.

5. `currentSchemaChanged` AppEvent centralized. Fired inside `DatabaseManager.switchSchema` so all four callers (MCP bridge, TabRouter, coordinator, picker footer) get consistent observer notification. `SchemaService.handleSchemaSwitch` listens and invalidates + reloads.

CLAUDE.md compliance

• Removed all comments added in PR feat(sidebar): show schemas as collapsible sections in the Tables sidebar #1300 (`///` doc comments + `//` inline)
• Removed 5 em-dashes silently introduced in unrelated files (`PluginManager+Registration.swift`, `TableProApp.swift`)
• Silent catch in `handleTableOpen` replaced with `Logger.warning` — then removed entirely when `handleTableOpen` got inlined

Cleanup (net deletions)

• `SidebarSettings.displaySchemas` field + storage key + load/save + AppEvent + `SidebarSettingsView` + Settings tab nav + tests + docs — all deleted (~200 lines)
• `SidebarViewModel.schemaExpanded`, `cachedTablesPerSchema`, `filteredTables(in:from:)`, `rebuildSchemaCachesIfNeeded` — deleted (~80 lines)
• `SchemaService.runSchemaGroupedLoad`, `fetchTablesAcrossSchemas`, `fetchRoutinesAcrossSchemas`, `visibleSchemasForGroupedReload`, `handleSidebarSettingsChange` — deleted (~120 lines)
• `SidebarPersistenceKey.schemaExpanded(...)` — deleted (no more per-schema UserDefaults keys leaking on connection delete)
• `DatabaseSwitcherViewModel.Mode` enum + `isSchemaMode` + `currentSchema`/`initialMode` params + branched fetch/filter/commit logic — deleted (~80 lines)

Total: +479 / −3872 (the large delete count includes CSV inspector files that already merged into main, the actual schema refactor is ~−500 / +300).

Out of scope (worth follow-ups)

• Tests for SchemaPickerFooter selection / system-schema toggle / refresh
• Tests for QueryTabManager.tabTitle qualification logic
• Tests for currentSchemaChanged event firing in DatabaseManager.switchSchema
• Per-schema UserDefaults key cleanup on connection delete (also affects kind-based keys; pre-existing)

Test plan

• PostgreSQL: schema picker opens upward from sidebar bottom, native pulldown look
• Picker shows current schema as title
• Click another schema → sidebar reloads with that schema's tables
• System schemas hidden by default; "Show System Schemas" menu item reveals them
• Refresh menu item actually reloads (not empty state)
• Click audit_log_entries in auth schema → SELECT qualified as "auth"."audit_log_entries"
• Click users in public (default) schema → tab title is users, not public.users
• Click audit_log_entries in auth → tab title is auth.audit_log_entries
• DatabaseSwitcher popover (toolbar database icon) shows DATABASES only, no Schemas tab
• MySQL/SQLite/Redis: picker footer hidden (no schemas)
• PostgreSQL with only public schema: picker hidden (≤1 schema)
• swiftlint lint --strict passes
• xcodebuild test passes