feat: chart support — make_chart tool with recharts rendering

Adds inline data visualisation to the chat agent. After a successful
run_query, the agent can call make_chart(chart_type, x, y, [group,
title, orientation]) and the result is rendered as a bar / line / area
/ pie chart inline in the chat thread, sourced from the previous query
result.

Backend (commands/chat.rs, models/chat.rs)
- New ChartConfig{chart_type, x, y, group?, title?, orientation?} model.
- New AgentAction::MakeChart{config} variant. Parser accepts both
  `chart_type` and the alternative `type` field name (qwen3 sometimes
  emits the latter). Validates chart_type is one of bar/line/area/pie.
- last_successful_query_result helper finds the most recent successful
  run_query in the working thread.
- MakeChart dispatcher: validates that x/y/group columns exist in the
  attached query result, emits a tool_result with the same QueryResult
  in `result` and the chart_config JSON in `text`. Mismatches surface
  as a clear error ("y column `name` is not in the last result.
  Available: company_name, legal_name, …").
- build_history compression unchanged: make_chart's tool_result text
  field (the small chart_config JSON) is included in LLM history; the
  large QueryResult.rows are NOT, since the per-tool branch only emits
  text for non-run_query tools.
- System prompt: documents make_chart with concrete usage hints
  (top-N → bar, time series → line/area, proportions → pie; skip for
  ≤2 or >500 rows). 7 new parser/dispatcher tests.

Frontend (src/components/chat/)
- recharts ^3.8 added.
- New ChartPreview component renders bar (vertical+horizontal), line,
  area, pie. Supports grouped series via the `group` config field by
  pivoting rows into a wide format. Y values coerced to numbers
  (parses strings, nulls → 0). Caps to 500 points to keep things
  responsive on huge results.
- ChatMessageView routes tool=="make_chart" tool_result through a new
  ChartToolResult that parses the config JSON from the message text
  and feeds the embedded QueryResult into ChartPreview.
- New labels/icons (BarChart3) and preview-extraction for make_chart
  in tool-call collapsed headers (`bar: carrier_name → trip_count`).

Verification: cargo test --lib 77 pass (+7), tsc clean, vitest 20
pass.

package.json

@@ -30,6 +30,7 @@
     "react": "^19.2.0",
     "react-dom": "^19.2.0",
     "react-resizable-panels": "^4.6.2",
+    "recharts": "^3.8.1",
     "sonner": "^2.0.7",
     "sql-formatter": "^15.7.0",
     "tailwind-merge": "^3.4.0",