Add a mobile-responsive layout and mirrored call controls

- Sidebar (channel list + nickname) becomes an off-canvas drawer below
  860px, opened via a new fixed top bar (hamburger + live status) and
  closed by tapping the backdrop or picking a channel.
- Mic/deafen/share get an always-visible bottom control bar so they're
  reachable mid-call without opening the drawer; updateMicButton/
  updateDeafenButton and their click wiring now target both the
  sidebar and mirror buttons via shared NodeLists instead of single
  elements.
- Per-user volume popup (previously hover-only) now also toggles on
  tap, and repositions below the row instead of to the side, which
  would overflow a narrow drawer.
- Screen-share tiles stack full-width, touch targets are enlarged,
  and the spatial map panel shrinks to leave room for the stage.

Also includes the working tree's prior swap of the spatial view from
Konva to Cytoscape.js (drag, scroll-to-zoom, fit-to-view, distance
edges) — not authored in this change, carried along since it was
already uncommitted.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

package-lock.json

@@ -16,7 +16,7 @@
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         "@opentelemetry/sdk-node": "^0.222.0",
         "@opentelemetry/semantic-conventions": "^1.43.0",
-        "konva": "^10.6.0",
+        "cytoscape": "^3.34.3",
         "matrix-js-sdk": "^42.4.0",
         "ws": "^8.18.0"
       },
@@ -3050,6 +3050,15 @@
         "url": "https://github.com/sponsors/ljharb"
       }
     },
+    "node_modules/cytoscape": {
+      "version": "3.34.3",
+      "resolved": "https://registry.npmjs.org/cytoscape/-/cytoscape-3.34.3.tgz",
+      "integrity": "sha512-yfYGhRcGAntq6YBD583j4n0Eg3jIxvWmZtz/5uz9UYkeIStSlMxuUja+ec5j3iBD8nv1rwaOAYMW09tBdkSeaQ==",
+      "license": "MIT",
+      "engines": {
+        "node": ">=0.10"
+      }
+    },
     "node_modules/data-uri-to-buffer": {
       "version": "4.0.1",
       "resolved": "https://registry.npmjs.org/data-uri-to-buffer/-/data-uri-to-buffer-4.0.1.tgz",
@@ -3941,38 +3950,6 @@
         "bignumber.js": "^9.0.0"
       }
     },
-    "node_modules/konva": {
-      "version": "10.6.0",
-      "resolved": "https://registry.npmjs.org/konva/-/konva-10.6.0.tgz",
-      "integrity": "sha512-6dyI91lZTx2mDu9DRo5zrT2Pg+Q3TFeyLfGQSAPJMhfvKjelbjPnTJimYQISam/EW+g0+pjLk/yztJKpSuxFPQ==",
-      "funding": [
-        {
-          "type": "patreon",
-          "url": "https://www.patreon.com/lavrton"
-        },
-        {
-          "type": "opencollective",
-          "url": "https://opencollective.com/konva"
-        },
-        {
-          "type": "github",
-          "url": "https://github.com/sponsors/lavrton"
-        }
-      ],
-      "license": "MIT",
-      "peerDependencies": {
-        "canvas": "^3.0.0",
-        "skia-canvas": "^3.0.0"
-      },
-      "peerDependenciesMeta": {
-        "canvas": {
-          "optional": true
-        },
-        "skia-canvas": {
-          "optional": true
-        }
-      }
-    },
     "node_modules/lightningcss": {
       "version": "1.33.0",
       "resolved": "https://registry.npmjs.org/lightningcss/-/lightningcss-1.33.0.tgz",