feat: adaptive video grid sizing (PR-C1)

Add computeGridSize helper in call.ts that computes optimal row/column
counts for a video tile grid given tile count and container dimensions,
maximizing tile size while maintaining ~16:9 cells.

Apply via computed grid-template-columns/rows on the multi-grid path
(no-spotlight) in VideoCallContent.svelte, replacing the fixed
grid-cols-1 / grid-cols-1 sm:grid-cols-2 heuristic. The multi-grid now
handles all tile counts from 1 to 12+ smoothly.

- computeGridSize(tileCount, containerWidth, containerHeight) -> GridSize
- Container dimensions bound via clientWidth/clientHeight
- useMultiGrid: dropped the > 2 check, now !useSpotlightLayout
- gridStyle: computed grid-template-columns/rows string from gridSize
- Scope: multi-grid only, no spotlight/strip/breakpoint changes

src/app/call.ts

@@ -31,6 +31,45 @@ export const LIVEKIT_PARTICIPANTS = 39004
 
 export {supportsAudioOutputSelection}
 
+export type GridSize = {rows: number; cols: number}
+
+/**
+ * Given a tile count and available container dimensions, returns the optimal
+ * number of rows and columns to maximize tile size while maintaining ~16:9
+ * cells. Handles 0 to 12+ tiles smoothly.
+ */
+export const computeGridSize = (
+  tileCount: number,
+  containerWidth: number,
+  containerHeight: number,
+): GridSize => {
+  if (tileCount <= 0) return {rows: 0, cols: 0}
+  if (tileCount === 1) return {rows: 1, cols: 1}
+
+  const TILE_ASPECT = 16 / 9
+  let best: GridSize = {rows: 1, cols: tileCount}
+  let bestArea = 0
+
+  // Guard against unmeasured container — fall back to a landscape-ish default
+  const w = containerWidth > 0 ? containerWidth : 800
+  const h = containerHeight > 0 ? containerHeight : 600
+
+  for (let cols = 1; cols <= tileCount; cols++) {
+    const rows = Math.ceil(tileCount / cols)
+    // Tile width constrained by both axes while maintaining 16:9
+    const tileWidth = Math.min(w / cols, (h / rows) * TILE_ASPECT)
+    const tileHeight = tileWidth / TILE_ASPECT
+    const area = tileWidth * tileHeight
+
+    if (area > bestArea) {
+      bestArea = area
+      best = {rows, cols}
+    }
+  }
+
+  return best
+}
+
 const LIVEKIT_DEFAULT_DEVICE_ID = "default"
 const VISUAL_SOURCES = [Track.Source.Camera, Track.Source.ScreenShare] as const
 const RECONNECT_DELAYS = [1000, 2000, 4000, 8000, 16000]

src/app/components/VideoCallContent.svelte

@@ -24,6 +24,8 @@
     participantMediaState,
     pubkeyFromLiveKitIdentity,
     videoTrackRevision,
+    computeGridSize,
+    type GridSize,
   } from "@app/call"
   import {profiles} from "@app/core"
 
@@ -173,10 +175,19 @@
     return videoTiles.filter(t => tileKey(t) !== pk)
   })
 
+  let gridWidth = $state(0)
+  let gridHeight = $state(0)
+
   const useSpotlightLayout = $derived(primaryTile !== undefined)
-  const useMultiGrid = $derived(!useSpotlightLayout && videoTiles.length > 2)
-  const multiGridClass = $derived(
-    layout === VideoCallLayout.Split ? "grid-cols-1" : "grid-cols-1 sm:grid-cols-2",
+  const useMultiGrid = $derived(!useSpotlightLayout)
+
+  const gridSize = $derived<GridSize>(
+    useMultiGrid ? computeGridSize(videoTiles.length, gridWidth, gridHeight) : {rows: 0, cols: 0},
+  )
+  const gridStyle = $derived(
+    gridSize.rows > 0 && gridSize.cols > 0
+      ? `grid-template-columns: repeat(${gridSize.cols}, 1fr); grid-template-rows: repeat(${gridSize.rows}, 1fr)`
+      : "",
   )
 
   $effect(() => {
@@ -283,7 +294,11 @@
         {/if}
       </div>
     {:else if useMultiGrid}
-      <div class={cx("grid min-h-0 flex-1 content-start gap-2 overflow-y-auto", multiGridClass)}>
+      <div
+        bind:clientWidth={gridWidth}
+        bind:clientHeight={gridHeight}
+        style={gridStyle}
+        class="grid min-h-0 flex-1 content-start gap-2 overflow-y-auto">
         {#each videoTiles as tile (tileKey(tile))}
           {@render videoTile(tile, "default")}
         {/each}