refactor: P0-P5 全面架构重构

P1 thiserror 类型化错误:
新增 core/src/error.rs QrError 枚举, 全链 String -> QrError,
包括 EmptyInput/InvalidVersion/DataTooLong/DecodeFail 等 8 种变体

P2 text_builder Tauri 统一:
新增 build_qr_text Tauri command, 删除前端 qrText.ts,
所有 mode 组件改为 invoke 调用 Rust 端构建文本

P3 QrConfig 颜色字段移除:
从 QrConfig/QrCode 移除 fg_color/bg_color,
改为 to_svg/to_image_bytes 参数传递

P4 前端 4 项合并:
Context 拆分为 StateContext+DispatchContext (H10),
新建 useModeForm 通用 hook (M11),
VCardMode grid-cols-2 网格布局 (M13),
persistHistory/loadHistory 迁至 utils/storage.ts (L9)

P5 算法优化:
MaskedView 懒计算替代 8 次 Matrix 克隆 (H9),
encoding_rs 精确 Kanji Shift JIS 映射 (H12)

验证: cargo check+clippy 通过, 81+24+7 全部测试通过

core/src/render/svg.rs

@@ -1,18 +1,12 @@
 use crate::qr::QrCode;
 
-pub fn render_svg(qr: &QrCode, logo: Option<&[u8]>) -> String {
+pub fn render_svg(qr: &QrCode, logo: Option<&[u8]>, fg: &[u8; 3], bg: &[u8; 3]) -> String {
     let matrix_size = qr.size() as u32;
     let margin = qr.margin as u32;
     let total = matrix_size + 2 * margin;
 
-    let fg = format!(
-        "#{:02X}{:02X}{:02X}",
-        qr.fg_color[0], qr.fg_color[1], qr.fg_color[2]
-    );
-    let bg = format!(
-        "#{:02X}{:02X}{:02X}",
-        qr.bg_color[0], qr.bg_color[1], qr.bg_color[2]
-    );
+    let fg_hex = format!("#{:02X}{:02X}{:02X}", fg[0], fg[1], fg[2]);
+    let bg_hex = format!("#{:02X}{:02X}{:02X}", bg[0], bg[1], bg[2]);
 
     let dark_count = qr
         .modules()
@@ -25,14 +19,14 @@ pub fn render_svg(qr: &QrCode, logo: Option<&[u8]>) -> String {
         r#"<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" width="{total}" height="{total}" viewBox="0 0 {total} {total}">"#
     ));
     svg.push_str(&format!(
-        r#"<rect width="{total}" height="{total}" fill="{bg}"/>"#
+        r#"<rect width="{total}" height="{total}" fill="{bg_hex}"/>"#
     ));
 
     for y in 0..matrix_size {
         for x in 0..matrix_size {
             if qr.modules()[y as usize][x as usize] {
                 svg.push_str(&format!(
-                    r#"<rect x="{}" y="{}" width="1" height="1" fill="{fg}"/>"#,
+                    r#"<rect x="{}" y="{}" width="1" height="1" fill="{fg_hex}"/>"#,
                     x + margin,
                     y + margin
                 ));