feat: rescope to AI-first DB harness with multi-DB chat agent

Removes enterprise/DBA features and replaces the marginal AI bar with a
central chat agent that has progressive-discovery tools, cross-session
memory, saved-query reuse, and inline result actions. Adds ClickHouse
support alongside PostgreSQL/Greenplum.

Cleanup
- Drop ~10k LOC of advanced features: Docker, Snapshots, Validation,
  Index Advisor, Role/User Management, Data Generator, ERD, Lookup.
- Trim deps: drop @xyflow/react, dagre, @types/dagre; cut tokio features
  to rt-multi-thread/sync/time/net/macros.
- Remove unused TuskError variants and dead helpers (topological_sort,
  invalidate_schema_cache).

Multi-DB (PostgreSQL + ClickHouse)
- New src-tauri/src/db/ module: ChClient (HTTP-based, reuses reqwest),
  sql_guard (cross-flavor read-only whitelist with 8 tests).
- ConnectionConfig gains db_flavor and secure fields with serde defaults
  for backwards-compatible connections.json.
- All connection/query/schema/data commands dispatch by flavor; CH
  covers connect, execute_query, list_databases/schemas/tables/views/
  columns/completion_schema, paginated table fetch.
- Frontend: dbCapabilities matrix, ConnectionDialog engine selector
  with port auto-swap and HTTPS toggle, SqlEditor switches to
  StandardSQL dialect for CH, TableDataView surfaces CH connections as
  read-only.

AI-first chat agent
- New src/components/chat/ panel with composer, message rendering,
  collapsible tool-call/result blocks, top-level ErrorBoundary.
- Backend agent loop in commands/chat.rs with strict-JSON tool
  protocol. Nine tools: list_databases, list_tables, get_columns,
  switch_database, run_query, remember, save_query, find_queries, final.
  Forgiving parser accepts both flat and nested-input shapes.
- Compressed history: only the last 4 run_query results carry sample
  rows (≤10, cells truncated to 200 chars) into LLM context; older
  results marked omitted.
- System prompt uses lite OVERVIEW (DB list + active-DB tables only)
  instead of full DDL — schema details are loaded on demand via
  get_columns. CH OVERVIEW shows cross-DB tables since CH allows
  db.table queries.

Cross-session memory (F1)
- Per-connection markdown file at app_data_dir/memory/<connection_id>.md,
  16KB cap with oldest-block eviction. Agent appends via remember()
  tool; the file is injected into LEARNED NOTES section of every system
  prompt.
- New Memory sidebar tab with editable textarea, badge for note count,
  empty-state with template. Edits picked up on the next agent turn.

Saved-query reuse (F2)
- Tools save_query and find_queries scoped to current connection.
  save_query attaches a UUID + timestamp; find_queries returns top 10
  matches with SQL preview ≤500 chars.
- Storage shared with the sidebar Saved panel.

Inline result actions (F3)
- run_query result block in chat gets Open-full (90vw × 80vh modal with
  full ResultsTable, no row cap) and Export (reuses ExportDialog for
  CSV/JSON via existing exportCsv/exportJson commands).

Verification
- cargo check clean, zero warnings.
- cargo test --lib: 50 pass (20 chat parser + 4 memory + 8 sql_guard +
  6 clean_sql + 12 escape_ident).
- npx tsc --noEmit clean.
- npx vitest run: 20 pass.

src/components/layout/Toolbar.tsx

@@ -7,7 +7,7 @@ import { ReadOnlyToggle } from "@/components/layout/ReadOnlyToggle";
 import { useAppStore } from "@/stores/app-store";
 import { useConnections, useReconnect } from "@/hooks/use-connections";
 import { toast } from "sonner";
-import { Database, Plus, RefreshCw, Search, Settings } from "lucide-react";
+import { Database, Plus, RefreshCw, Settings, Sparkles } from "lucide-react";
 import type { ConnectionConfig, Tab } from "@/types";
 import { getEnvironment } from "@/lib/environment";
 import { AppSettingsSheet } from "@/components/settings/AppSettingsSheet";
@@ -45,12 +45,12 @@ export function Toolbar() {
     addTab(tab);
   };
 
-  const handleNewLookup = () => {
+  const handleNewChat = () => {
     if (!activeConnectionId) return;
     const tab: Tab = {
       id: crypto.randomUUID(),
-      type: "lookup",
-      title: "Entity Lookup",
+      type: "chat",
+      title: "Chat",
       connectionId: activeConnectionId,
       database: currentDatabase ?? undefined,
     };
@@ -101,22 +101,22 @@ export function Toolbar() {
           variant="ghost"
           size="xs"
           className="gap-1.5 text-muted-foreground hover:text-foreground"
-          onClick={handleNewQuery}
+          onClick={handleNewChat}
           disabled={!activeConnectionId}
         >
-          <Plus className="h-3.5 w-3.5" />
-          <span className="text-xs font-medium">New Query</span>
+          <Sparkles className="h-3.5 w-3.5" />
+          <span className="text-xs font-medium">Ask AI</span>
         </Button>
 
         <Button
           variant="ghost"
           size="xs"
           className="gap-1.5 text-muted-foreground hover:text-foreground"
-          onClick={handleNewLookup}
+          onClick={handleNewQuery}
           disabled={!activeConnectionId}
         >
-          <Search className="h-3.5 w-3.5" />
-          <span className="text-xs font-medium">Lookup</span>
+          <Plus className="h-3.5 w-3.5" />
+          <span className="text-xs font-medium">New Query</span>
         </Button>
 
         <div className="flex-1" />