feat(player): add "Xoá đánh dấu" button to clear marks without regen

Lets the player wipe all crossed cells on the current card without
generating a new grid. Shown next to "Tạo bảng mới" only when a grid
exists. Confirms before clearing if any cell is marked.

- src/lib/PlayerBoard.svelte: handleClear + secondary outline button
- docs/codebase-summary.md, project-overview-pdr.md: reflect new action

web/docs/codebase-summary.md

@@ -11,7 +11,7 @@
 ### Shared Components
 | File | Purpose |
 |------|---------|
-| `src/lib/PlayerBoard.svelte` | Reusable player card (9×9 grid rendered as 3 stacked 3×9 mini-cards: Tân Tân / An khang thịnh vượng / Tân Tân tốt nhất). Tall (3:5 on mobile; wider on sm+) cells with condensed bold black numbers (`tan-tan-num` font stack), white number cells, purple empty cells by default. Handles crossed state, bingo popup, "Chờ X" toast. Accepts `storagePrefix` prop for multi-card isolation. Empty cells use `--empty-cell-bg` CSS var from settings store. |
+| `src/lib/PlayerBoard.svelte` | Reusable player card (9×9 grid rendered as 3 stacked 3×9 mini-cards: Tân Tân / An khang thịnh vượng / Tân Tân tốt nhất). Tall (3:5 on mobile; wider on sm+) cells with condensed bold black numbers (`tan-tan-num` font stack), white number cells, purple empty cells by default. Handles crossed state, bingo popup, "Chờ X" toast. Two header actions: "Tạo bảng mới" (regenerate grid) and "Xoá đánh dấu" (clear all marks, keep grid — only shown when a grid exists). Accepts `storagePrefix` prop for multi-card isolation. Empty cells use `--empty-cell-bg` CSS var from settings store. |
 | `src/lib/SettingsButton.svelte` | Gear icon + modal. 4 fieldsets: Giao diện (theme auto/light/dark), Chế độ quản trò (master mode toggle), Tự động xổ (auto-call + speed 1–10s), Màu ô trống (10 Excel color swatches). Reset-to-default button. Mounted on `/`. |
 | `src/lib/MasterPanel.svelte` | Host controls. New game / draw, "Số vừa xổ" hero token, "Thứ tự đã xổ" history list, 11×9 last-digit-aligned tracking grid (with circular tokens + draw-order overlay). No host's own player card (the player already has one above). "Xổ số" / "Bắt đầu / Dừng" button bound to auto-call. Mounted conditionally on `/` when `settings.masterMode === true`. |
 | `src/lib/PageFooter.svelte` | Footer with tagline ("Made by miti99 with ❤️ SVG icon") + link. Mounted on `/`. |

web/docs/project-overview-pdr.md

@@ -88,6 +88,7 @@ State is entirely client-side. Each card / panel instance uses a unique localSto
 - [x] Player card is 9×9 with **exactly 5 per row and 5 per column**.
 - [x] Numbers within each column are **ascending top-to-bottom**.
 - [x] Player can click cells to toggle crossed state.
+- [x] Player can clear all marks on the current card without regenerating it (confirm prompt when marks exist).
 - [x] Bingo popup triggers when row is complete, shows row number and "Kinh!" message.
 - [x] Player may keep marking after a Kinh — no game-end lock.
 - [x] Toast notifications show "Chờ X" before bingo (one number remaining).

web/src/lib/PlayerBoard.svelte

@@ -120,6 +120,17 @@
     dismissToast();
   }
 
+  function handleClear() {
+    if (!grid) return;
+    const hasMarks = crossed.some((row) => row.some(Boolean));
+    if (hasMarks && !confirm("Bạn có muốn xoá tất cả đánh dấu không?")) return;
+    crossed = grid.map((row) => row.map(() => false));
+    celebratedRows.clear();
+    notifiedWaitingRows.clear();
+    dismissToast();
+    showCongrats = false;
+  }
+
   /**
    * @param {number} row
    * @param {number} col
@@ -146,7 +157,7 @@
   ];
 </script>
 
-<div class="flex justify-center mb-6">
+<div class="flex flex-wrap justify-center gap-3 mb-6">
   <button
     onclick={handleGenerate}
     class="px-8 py-3 rounded-full font-semibold text-white
@@ -156,6 +167,19 @@
   >
     Tạo bảng mới
   </button>
+  {#if grid}
+    <button
+      onclick={handleClear}
+      class="px-6 py-3 rounded-full font-semibold
+             text-slate-700 dark:text-slate-200
+             bg-white dark:bg-slate-800
+             border-2 border-slate-300 dark:border-slate-600
+             hover:bg-slate-50 dark:hover:bg-slate-700
+             active:scale-95 transition-all"
+    >
+      Xoá đánh dấu
+    </button>
+  {/if}
 </div>
 
 {#if grid}