refactor(sidebar): replace schema-grouped tree with picker footer (TablePlus pattern)

[datlechin]

Layers on top of the merged #1300 to address bugs and replace the schema-grouped tree UX with a TablePlus-style schema picker footer. Original work on #1307 was opened against pre-#1300 main; this is the version rebased onto current main.

Why this refactor

#1300 added schemas-as-collapsible-sections in the sidebar tree. Real issues from testing:

• Cross-schema SELECT was unqualified. Clicking `auth.audit_log_entries` produced `SELECT * FROM "audit_log_entries"` because `TableSelectionAction.navigate` decomposed TableInfo into `(tableName, isView)` and dropped the schema. PG's search_path didn't include `auth` → "relation does not exist".
• TableInfo identity regression for ALL users, not just grouped-mode. `PluginDriverAdapter.fetchTables()` over-applied `schema: table.schema ?? resolvedSchema` → TableInfo.id changed from `users_TABLE` to `public.users_TABLE`, breaking selection sets, tab restoration, ER Diagram layouts.
• Schema fan-out: 3 * N concurrent queries on a single connection (`runSchemaGroupedLoad` parallelized tables + procedures + functions per schema).
• Tree of all schemas doesn't scale. Supabase has 8+ schemas (auth, storage, realtime, extensions, graphql, pgsodium, public, vault) — the tree becomes noise. MSSQL enterprise hits 50+.
• Conflates two concepts: "browse schemas" (the tree implied) vs "switch active schema" (what users actually want).

What this PR does

Replaces the tree with a TablePlus-style picker footer:

1. `SchemaPickerFooter` at sidebar bottom — native `NSPopUpButton(pullsDown: true)` via `NSViewRepresentable`. Real HIG bezel, chevron-on-right, opens upward as a real `NSMenu`. System schemas hidden by default; "Show System Schemas" toggle menu item. Footer hidden when ≤ 1 schema or driver doesn't support schemas.

2. Sidebar shows ONE active schema at a time — flat by capability (Tables / Views / Procedures / Functions), same as today's flat mode.

3. DatabaseSwitcher popover loses its schema mode — used to have a segmented control toggling Databases/Schemas. Databases-only now; schemas live in the picker footer.

4. `AppEvents.currentSchemaChanged: PassthroughSubject<UUID, Never>` fired centrally in `DatabaseManager.switchSchema`. All four callers (MCP bridge, TabRouter, coordinator, picker footer) get consistent observer notification. `SchemaService.handleSchemaSwitch` listens and invalidates + reloads.

Fixes the bugs PR #1300 had:

5. Cross-schema SELECT qualifies properly. `openTableTab(_ table: TableInfo)` canonical entry forwards to `addTableTab(name, schema:, ...)` → `QueryTab.buildBaseTableQuery(schemaName:)` → qualified SQL. Three click paths fixed: double-click (`onDoubleClick`), single-click (`TableSelectionAction.navigate(table: TableInfo)`), new-window (`EditorTabPayload` → `SessionStateFactory`).

6. Tab title shows `schema.tableName` only when schema differs from the connection's default schema. `users` from `public` stays clean; `auth.audit_log_entries` shows the schema for disambiguation.

7. TableInfo identity preserved. `PluginDriverAdapter.fetchTables()` doesn't apply schema fallback (preserves nil-passthrough). Only `fetchTables(schema:)` does.

8. `TableSelectionAction.navigate(table: TableInfo)` carries the whole value instead of decomposing — eliminates the bug class.

Deletions

• `SidebarSettings.displaySchemas` infrastructure (settings field, storage key, AppEvent, view, tests, docs) — ~200 lines
• `SidebarViewModel.schemaExpanded`, `cachedTablesPerSchema`, `filteredTables(in:from:)`, `rebuildSchemaCachesIfNeeded` — ~80 lines
• `SchemaService.runSchemaGroupedLoad`, `fetchTablesAcrossSchemas`, `fetchRoutinesAcrossSchemas`, `visibleSchemasForGroupedReload`, `handleSidebarSettingsChange` — ~120 lines
• `SidebarPersistenceKey.schemaExpanded(...)` — no more per-schema UserDefaults keys leaking
• `DatabaseSwitcherViewModel.Mode` + `isSchemaMode` + schema fetch/filter/commit branches — ~80 lines

Net: substantial code reduction. Architecture simplifies because there's now ONE schema model (current = active) instead of two (current + visible-in-tree).

CLAUDE.md compliance

• No comments added in code paths (per CLAUDE.md mandate)
• No em-dashes in user-facing strings
• No banned words (seamless, robust, comprehensive, etc.)
• `String(localized:)` used correctly throughout

Test plan

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