refactor: split App.vue into composables and layout components

Extract 11 composables and 5 layout components from App.vue, reducing it
from 1921 to 327 lines. Unify dialog overlay style across DialogContent
and DialogScrollContent for consistent backdrop appearance.

src/composables/useFileDrop.ts

@@ -0,0 +1,90 @@
+import { useI18n } from "vue-i18n";
+import { useConnectionStore } from "@/stores/connectionStore";
+import { useQueryStore } from "@/stores/queryStore";
+import { useToast } from "@/composables/useToast";
+import * as api from "@/lib/api";
+import type { ConnectionConfig } from "@/types/database";
+
+const DB_EXTENSIONS = [".db", ".sqlite", ".sqlite3", ".duckdb"];
+
+function getDbType(path: string): "sqlite" | "duckdb" | null {
+  const lower = path.toLowerCase();
+  if (lower.endsWith(".duckdb")) return "duckdb";
+  if (DB_EXTENSIONS.some((ext) => lower.endsWith(ext))) return "sqlite";
+  return null;
+}
+
+function getDataFileQuery(path: string): string | null {
+  const lower = path.toLowerCase();
+  const escaped = path.replace(/'/g, "''");
+  if (lower.endsWith(".parquet")) return `SELECT * FROM read_parquet('${escaped}') LIMIT 1000`;
+  if (lower.endsWith(".csv")) return `SELECT * FROM read_csv('${escaped}') LIMIT 1000`;
+  if (lower.endsWith(".tsv")) return `SELECT * FROM read_csv('${escaped}', delim='\\t') LIMIT 1000`;
+  if (lower.endsWith(".json")) return `SELECT * FROM read_json('${escaped}') LIMIT 1000`;
+  return null;
+}
+
+export function useFileDrop() {
+  const { t } = useI18n();
+  const connectionStore = useConnectionStore();
+  const queryStore = useQueryStore();
+  const { toast } = useToast();
+
+  async function setupFileDrop() {
+    const { getCurrentWebview } = await import("@tauri-apps/api/webview");
+    const webview = getCurrentWebview();
+    await webview.onDragDropEvent(async (event) => {
+      if (event.payload.type !== "drop") return;
+      for (const path of event.payload.paths) {
+        const name = path.split("/").pop()?.split("\\").pop() || path;
+
+        const dataQuery = getDataFileQuery(path);
+        if (dataQuery) {
+          const config: ConnectionConfig = {
+            id: crypto.randomUUID(),
+            name: `[Preview] ${name}`,
+            db_type: "duckdb",
+            driver_profile: "duckdb",
+            driver_label: "DuckDB",
+            url_params: "",
+            host: ":memory:",
+            port: 0,
+            username: "",
+            password: "",
+          };
+          const connectionId = await api.connectDb(config);
+          connectionStore.addEphemeralConnection({ ...config, id: connectionId });
+          const tabId = queryStore.createTab(connectionId, "", name, "query");
+          queryStore.updateSql(tabId, dataQuery);
+          queryStore.executeCurrentTab();
+          toast(t("welcome.fileOpened", { name }));
+          continue;
+        }
+
+        const dbType = getDbType(path);
+        if (!dbType) continue;
+        const config: ConnectionConfig = {
+          id: crypto.randomUUID(),
+          name,
+          db_type: dbType,
+          driver_profile: dbType,
+          driver_label: dbType === "duckdb" ? "DuckDB" : "SQLite",
+          url_params: "",
+          host: path,
+          port: 0,
+          username: "",
+          password: "",
+        };
+        try {
+          await connectionStore.addConnection(config);
+          void connectionStore.connect(config);
+          toast(t("welcome.fileOpened", { name }));
+        } catch (e: any) {
+          toast(t("connection.saveFailed", { message: e?.message || String(e) }), 5000);
+        }
+      }
+    });
+  }
+
+  return { setupFileDrop };
+}