fix: handle PG INTERVAL type, robust compact LLM output + feedback

INTERVAL handling
- pg_value_to_json now decodes PG INTERVAL via PgInterval and renders
  it psql-style: `1 year 2 mons 3 days 04:05:06`. Previously
  AVG(timestamp - timestamp) and similar interval-returning queries
  showed `<unsupported type: INTERVAL>` in chat results.
- 7 unit tests covering zero, days-only, mixed, negative, microsecond
  fraction, and the singular/plural unit rules.

Compact reliability
- Sharper system prompt: explicitly instructs plain text starting with
  `-`, no JSON, no fences, no field names. qwen3-coder is heavily
  trained on the agent JSON protocol and was sometimes returning
  `{"action":"final","text":"..."}` even for the compact prompt.
- New clean_summary helper strips ``` fences (with or without lang
  identifier) and extracts the underlying string from a JSON envelope
  if the model still wraps the answer (looks for text/summary/content/
  answer/output keys). 6 unit tests.
- Frontend useChat.compact: success/no-op/error toasts via sonner so
  the user sees what happened. "Nothing to compact" appears when there
  is no older history beyond the last user turn (previously silent).

Verification: cargo test --lib 66 pass (+13), tsc clean, vitest 20
pass.

src/hooks/use-chat.ts

@@ -1,4 +1,5 @@
 import { useCallback } from "react";
+import { toast } from "sonner";
 import { chatCompact, chatSend } from "@/lib/tauri";
 import { useAppStore } from "@/stores/app-store";
 import type { ChatMessage } from "@/types";
@@ -24,22 +25,40 @@ export function useChat(tabId: string, connectionId: string) {
 
   const compact = useCallback(async (): Promise<boolean> => {
     const state = useAppStore.getState();
-    if (state.chatPending[tabId]) return false;
+    if (state.chatPending[tabId]) {
+      toast.message("Wait for the agent to finish first.");
+      return false;
+    }
     const history = state.chatThreads[tabId] ?? [];
-    if (history.length === 0) return false;
+    if (history.length === 0) {
+      toast.message("Nothing to compact yet.");
+      return false;
+    }
 
+    const beforeCount = history.length;
     setChatPending(tabId, true);
     try {
       const turn = await chatCompact(connectionId, history);
+      const afterCount = turn.messages.length;
+      // Backend returns the same thread untouched when there's nothing older
+      // than the last user turn; surface that instead of silently no-op.
+      if (afterCount >= beforeCount) {
+        toast.message("Nothing to compact (no older history beyond the last question).");
+        return false;
+      }
       replaceChatThread(tabId, turn.messages);
       setChatUsage(tabId, turn.usage);
+      const removed = beforeCount - afterCount + 1; // +1: original older replaced by single summary
+      toast.success(`Compacted ${removed} earlier message${removed === 1 ? "" : "s"}.`);
       return true;
     } catch (err) {
+      const text = `Compact failed: ${String(err)}`;
+      toast.error("Compact failed", { description: String(err) });
       appendChatMessages(tabId, [
         {
           id: newId("err"),
           role: "assistant",
-          text: `Compact failed: ${String(err)}`,
+          text,
           created_at: Date.now(),
         },
       ]);