fix(web): sticky header, correct bottom-corner suit glyphs, main min-width

- header: position sticky so HUD/controls never scroll off screen
- .card .corner.bottom: remove rotate(180deg) — ♠ rotated looks like ♥,
  causing players to misread suit on the bottom corner
- main: add min-width:0 so flex container doesn't push board off-edge

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

solitaire_server/web/game.js

@@ -196,7 +196,7 @@ function render(s) {
 
     hudScore.textContent = `Score: ${s.score}`;
     hudMoves.textContent = `Moves: ${s.move_count}`;
-    hudStock.textContent = `Stock: ${s.stock.length}`;
+    if (hudStock) hudStock.textContent = `Stock: ${s.stock.length}`;
     btnUndo.disabled     = s.undo_stack_len === 0;
 
     const visible = new Map();