feat(ui): 首次启动加密引导 + 真窗口图标 (.syso)

首次启动:
- 配置文件不存在时弹欢迎对话框：选「启用密码保护」或「稍后再说」
- 选启用直接进 setPasswordDialog，OK 时 store.SetPassword 触发首次保存
- io/fs.ErrNotExist 检测新分支
- modalCard 抽出 modalShell 内核以便 welcomeDialog 自定义 footer

真窗口图标:
- tools/gen_ico 从 PNG 生成多分辨率 .ico（7 尺寸：16~256）
- 提交 rsrc_windows.syso（rsrc 通过 goproxy.cn 生成），go build 自动链入
- tray_windows.go 改用嵌入资源 ID=1，失败 fallback IDI_APPLICATION
- README 更新完整的图标生成流程

tools/gen_ico/main.go

@@ -0,0 +1,144 @@
+// gen_ico produces a multi-resolution .ico file from a single PNG
+// source. The output embeds each size as a PNG payload (Vista+ .ico
+// format) so the result stays small and crisp.
+//
+// Usage:
+//
+//	go run ./tools/gen_ico [src.png] [dst.ico]
+//
+// Defaults:
+//
+//	src = internal/ui/assets/app_icon.png
+//	dst = internal/ui/assets/app_icon.ico
+//
+// After generating the .ico, embed it into the Windows binary via:
+//
+//	go run github.com/akavel/rsrc@latest \
+//	    -ico internal/ui/assets/app_icon.ico \
+//	    -o internal/ui/assets/rsrc_windows.syso
+//
+// rsrc_windows.syso is auto-linked by `go build` on Windows.
+package main
+
+import (
+	"bytes"
+	"encoding/binary"
+	"fmt"
+	"image"
+	"image/png"
+	"io"
+	"os"
+
+	"golang.org/x/image/draw"
+)
+
+// icoSizes are the embedded resolutions. Windows picks the largest size
+// that fits the target use (title bar = 16, Alt-Tab = 32, Explorer = up
+// to 256) and scales down. Skipping sizes Windows can't read anyway
+// keeps the file under 200 KB.
+var icoSizes = []int{16, 24, 32, 48, 64, 128, 256}
+
+// icoEntry holds one image's dimensions and its encoded PNG bytes.
+type icoEntry struct {
+	w, h int
+	png  []byte
+}
+
+func main() {
+	srcPath := "internal/ui/assets/app_icon.png"
+	dstPath := "internal/ui/assets/app_icon.ico"
+	if len(os.Args) > 1 {
+		srcPath = os.Args[1]
+	}
+	if len(os.Args) > 2 {
+		dstPath = os.Args[2]
+	}
+	if err := run(srcPath, dstPath); err != nil {
+		fmt.Fprintln(os.Stderr, "gen_ico:", err)
+		os.Exit(1)
+	}
+}
+
+func run(srcPath, dstPath string) error {
+	srcF, err := os.Open(srcPath)
+	if err != nil {
+		return fmt.Errorf("open src: %w", err)
+	}
+	defer srcF.Close()
+	src, _, err := image.Decode(srcF)
+	if err != nil {
+		return fmt.Errorf("decode src: %w", err)
+	}
+
+	var entries []icoEntry
+	for _, size := range icoSizes {
+		dst := image.NewNRGBA(image.Rect(0, 0, size, size))
+		// CatmullRom gives sharper results at small sizes than the
+		// default nearest-neighbour.
+		draw.CatmullRom.Scale(dst, dst.Bounds(), src, src.Bounds(), draw.Over, nil)
+
+		var buf bytes.Buffer
+		if err := png.Encode(&buf, dst); err != nil {
+			return fmt.Errorf("encode %dpx: %w", size, err)
+		}
+		entries = append(entries, icoEntry{w: size, h: size, png: buf.Bytes()})
+	}
+
+	out, err := os.Create(dstPath)
+	if err != nil {
+		return fmt.Errorf("create dst: %w", err)
+	}
+	defer out.Close()
+	return writeICO(out, entries)
+}
+
+// writeICO serialises a Vista+ PNG-based .ico file: ICONDIR + one
+// ICONDIRENTRY per image + the PNG bytes concatenated.
+func writeICO(w io.Writer, entries []icoEntry) error {
+	if _, err := w.Write([]byte{
+		0, 0, // reserved
+		1, 0, // type = 1 (icon)
+		byte(len(entries)), 0, // count
+	}); err != nil {
+		return err
+	}
+
+	const dirHdrLen = 6
+	const dirEntryLen = 16
+	headerLen := dirHdrLen + dirEntryLen*len(entries)
+
+	offset := headerLen
+	for _, e := range entries {
+		wByte := byte(e.w)
+		if e.w >= 256 {
+			// 0 means 256 in the .ico directory.
+			wByte = 0
+		}
+		hByte := byte(e.h)
+		if e.h >= 256 {
+			hByte = 0
+		}
+		size := uint32(len(e.png))
+
+		entry := make([]byte, dirEntryLen)
+		entry[0] = wByte
+		entry[1] = hByte
+		entry[2] = 0 // color palette
+		entry[3] = 0 // reserved
+		binary.LittleEndian.PutUint16(entry[4:], 1)  // planes
+		binary.LittleEndian.PutUint16(entry[6:], 32) // bit count
+		binary.LittleEndian.PutUint32(entry[8:], size)
+		binary.LittleEndian.PutUint32(entry[12:], uint32(offset))
+		if _, err := w.Write(entry); err != nil {
+			return err
+		}
+		offset += int(size)
+	}
+
+	for _, e := range entries {
+		if _, err := w.Write(e.png); err != nil {
+			return err
+		}
+	}
+	return nil
+}