Premium publishing: support 'deck' template mode

A user trying to publish a premium slide deck hit
"Premium preview not supported for mode 'deck'." — the publish
flow couldn't render the watermarked preview because the
mode-dispatch in _renderModeToExportCanvas had cases for
cover / disc / jewel / customart / slide / pixelart /
disc-design but not deck.

Fix:

- Add a deck branch to _premiumPreviewLongAxisInches that pulls
  width / height from state.deck.slides[0].slide (matches the
  customart / slide path: pixels → inches at 96 dpi).
- Add a deck branch to _renderModeToExportCanvas that dispatches
  to a new renderDeckToExportCanvas.
- renderDeckToExportCanvas mirrors renderCustomArtToExportCanvas
  but pulls the slide payload from the deck. Uses the currently-
  edited slide when deck.editingIndex is set (matches what the
  user sees while composing); otherwise falls back to slide 1
  (the deck-thumbnail convention). composeSlideForDeck applies
  the deck theme so the preview shows the buyer what they'll
  actually unlock. Empty decks emit a blank canvas rather than
  throwing — the publish modal already gates on having content,
  so this is just a defensive fallback.

With this commit every template mode supports premium
publishing — including decks — so a creator can charge sats for
a slide deck the same way they can for a cover or custom art.

Decryption side is mode-agnostic (Premium.decryptPayload just
returns the JSON, which loadProjectFromText already routes to
the deck builder), so no buyer-side change needed.

src/index.html

@@ -17846,6 +17846,15 @@
         const h = (state.customArt?.height || 1080) / 96;
         return Math.max(w, h);
       }
+      if (mode === 'deck') {
+        // The preview renders the deck's first slide. Deck slides
+        // default to 1920x1080 = 20x11.25in like customart / slide.
+        const slides = state.deck?.slides || [];
+        const sl = slides[0]?.slide || {};
+        const w = (Number(sl.width) || 1920) / 96;
+        const h = (Number(sl.height) || 1080) / 96;
+        return Math.max(w, h);
+      }
       // disc-sheet, jewel: letter-sized
       return 11;
     }
@@ -17857,9 +17866,50 @@
       if (mode === 'jewel') return renderJewelSheetToExportCanvas(dpi);
       if (mode === 'customart' || mode === 'slide') return renderCustomArtToExportCanvas(dpi);
       if (mode === 'pixelart') return renderPixelArtToExportCanvas();
+      if (mode === 'deck') return renderDeckToExportCanvas(dpi);
       throw new Error(`Premium preview not supported for mode "${mode}".`);
     }
 
+    // Render the deck's first slide (themed via composeSlideForDeck)
+    // as the premium preview source. This is the slide buyers see
+    // before unlocking — using slide 1 matches the deck-thumbnail
+    // convention. Decks with zero slides produce an empty canvas
+    // rather than throwing so the publish modal can still proceed.
+    async function renderDeckToExportCanvas(dpi) {
+      const deck = state.deck || {};
+      const slides = Array.isArray(deck.slides) ? deck.slides : [];
+      // Prefer the currently-edited slide if the user is mid-edit;
+      // otherwise slide 1.
+      let entry = null;
+      if (deck.editingIndex != null && slides[deck.editingIndex]) {
+        entry = slides[deck.editingIndex];
+      } else if (slides[0]) {
+        entry = slides[0];
+      }
+      if (!entry) {
+        const canvas = document.createElement('canvas');
+        setupExportCanvas(canvas, 20, 11.25, dpi);
+        return canvas;
+      }
+      const composed = composeSlideForDeck(entry, deck.theme || {});
+      const sl = composed.slide || {};
+      const wIn = (Number(sl.width) || 1920) / 96;
+      const hIn = (Number(sl.height) || 1080) / 96;
+      const canvas = document.createElement('canvas');
+      const ctx = setupExportCanvas(canvas, wIn, hIn, dpi);
+      const bg = sl.background || '#ffffff';
+      if (bg && bg.toLowerCase() !== '#ffffff' && bg.toLowerCase() !== '#fff') {
+        ctx.fillStyle = bg;
+        ctx.fillRect(0, 0, canvas.width, canvas.height);
+      }
+      const layers = (composed.layers || [])
+        .filter(l => (l.target === 'canvas' || !l.target) && !l.hidden && !l._generatedDiscLayer)
+        .slice()
+        .sort((a, b) => (a.z || 0) - (b.z || 0));
+      for (const layer of layers) await drawLayerToCanvas(ctx, layer, dpi);
+      return canvas;
+    }
+
     // Render the standalone pixel-art design (state.pixelArt frame 0) into
     // a canvas suitable for the premium watermarked preview. Native pixel
     // dimensions are tiny (32-128 px), so scale up by the largest integer