Virtualize the message list

A conversation rendered every message it had loaded. Nothing bounded that
work, so cost grew with history: at 2000 messages the list mounted ~20k DOM
nodes, took ~285ms to mount, and ~212ms to re-render on any update. While
that runs, nothing else on the main thread gets a turn, typing included.

Only the visible window is mounted now, measured in Chromium with the real
component tree:

  history    mount            re-render        DOM nodes
    2000     285ms -> 4ms     212ms -> 2.2ms   22038 -> 225

Rendered rows stay at ~17 regardless of how much history is loaded.

Row heights are measured rather than assumed, since messages vary from one line
to a wrapped paragraph with path badges, so the scroll behaviors that used
to do height arithmetic now anchor by index instead, which is exact even
when off-screen rows still carry estimated heights:

- opening a conversation, and following new messages, anchor to the last row
- loading older messages anchors to the message that was on top
- jump-to-message and jump-to-unread scroll by index, since the target row
  may not be mounted yet

Two degenerate cases are handled explicitly: a viewport that measures zero
(before first layout, hidden tab) falls back to a nominal height instead of
collapsing the window to nothing, and a row that measures zero keeps its
estimate instead of ballooning the window.

Most of the diff is re-indentation from the new wrapper element; the real
change is 112 added / 36 removed lines (git diff -w).

frontend/package-lock.json

@@ -1,12 +1,12 @@
 {
   "name": "remoteterm-meshcore-frontend",
-  "version": "3.16.0",
+  "version": "3.17.0",
   "lockfileVersion": 3,
   "requires": true,
   "packages": {
     "": {
       "name": "remoteterm-meshcore-frontend",
-      "version": "3.16.0",
+      "version": "3.17.0",
       "dependencies": {
         "@codemirror/lang-python": "^6.2.1",
         "@codemirror/theme-one-dark": "^6.1.3",
@@ -17,6 +17,7 @@
         "@radix-ui/react-separator": "^1.1.8",
         "@radix-ui/react-slot": "^1.2.4",
         "@radix-ui/react-tabs": "^1.1.13",
+        "@tanstack/react-virtual": "^3.14.8",
         "@uiw/react-codemirror": "^4.25.4",
         "class-variance-authority": "^0.7.1",
         "clsx": "^2.1.1",
@@ -1543,9 +1544,6 @@
         "arm64"
       ],
       "dev": true,
-      "libc": [
-        "glibc"
-      ],
       "license": "MIT",
       "optional": true,
       "os": [
@@ -1563,9 +1561,6 @@
         "arm64"
       ],
       "dev": true,
-      "libc": [
-        "musl"
-      ],
       "license": "MIT",
       "optional": true,
       "os": [
@@ -1583,9 +1578,6 @@
         "ppc64"
       ],
       "dev": true,
-      "libc": [
-        "glibc"
-      ],
       "license": "MIT",
       "optional": true,
       "os": [
@@ -1603,9 +1595,6 @@
         "s390x"
       ],
       "dev": true,
-      "libc": [
-        "glibc"
-      ],
       "license": "MIT",
       "optional": true,
       "os": [
@@ -1623,9 +1612,6 @@
         "x64"
       ],
       "dev": true,
-      "libc": [
-        "glibc"
-      ],
       "license": "MIT",
       "optional": true,
       "os": [
@@ -1643,9 +1629,6 @@
         "x64"
       ],
       "dev": true,
-      "libc": [
-        "musl"
-      ],
       "license": "MIT",
       "optional": true,
       "os": [
@@ -1744,6 +1727,33 @@
       "integrity": "sha512-e7Mew686owMaPJVNNLs55PUvgz371nKgwsc4vxE49zsODpJEnxgxRo2y/OKrqueavXgZNMDVj3DdHFlaSAeU8g==",
       "license": "MIT"
     },
+    "node_modules/@tanstack/react-virtual": {
+      "version": "3.14.8",
+      "resolved": "https://registry.npmjs.org/@tanstack/react-virtual/-/react-virtual-3.14.8.tgz",
+      "integrity": "sha512-O39GJQpAYEJcIu3uN1//YtmhjSEOyw75vg9CKCatBDPiD5hKtZQoJHfferyrB/LdOD3UWaoMLWtdEjarwIwdDw==",
+      "license": "MIT",
+      "dependencies": {
+        "@tanstack/virtual-core": "3.17.6"
+      },
+      "funding": {
+        "type": "github",
+        "url": "https://github.com/sponsors/tannerlinsley"
+      },
+      "peerDependencies": {
+        "react": "^16.8.0 || ^17.0.0 || ^18.0.0 || ^19.0.0",
+        "react-dom": "^16.8.0 || ^17.0.0 || ^18.0.0 || ^19.0.0"
+      }
+    },
+    "node_modules/@tanstack/virtual-core": {
+      "version": "3.17.6",
+      "resolved": "https://registry.npmjs.org/@tanstack/virtual-core/-/virtual-core-3.17.6.tgz",
+      "integrity": "sha512-h0/Ebo18CkOrChlQIhNtQkM5ySUnh/GumQ/D1st3hG2HWUPEF+ILUc2k29UtivCi/9G7w7G3/f7Xyd5cCFbKBw==",
+      "license": "MIT",
+      "funding": {
+        "type": "github",
+        "url": "https://github.com/sponsors/tannerlinsley"
+      }
+    },
     "node_modules/@testing-library/dom": {
       "version": "10.4.1",
       "resolved": "https://registry.npmjs.org/@testing-library/dom/-/dom-10.4.1.tgz",
@@ -4504,9 +4514,6 @@
         "arm64"
       ],
       "dev": true,
-      "libc": [
-        "glibc"
-      ],
       "license": "MPL-2.0",
       "optional": true,
       "os": [
@@ -4528,9 +4535,6 @@
         "arm64"
       ],
       "dev": true,
-      "libc": [
-        "musl"
-      ],
       "license": "MPL-2.0",
       "optional": true,
       "os": [
@@ -4552,9 +4556,6 @@
         "x64"
       ],
       "dev": true,
-      "libc": [
-        "glibc"
-      ],
       "license": "MPL-2.0",
       "optional": true,
       "os": [
@@ -4576,9 +4577,6 @@
         "x64"
       ],
       "dev": true,
-      "libc": [
-        "musl"
-      ],
       "license": "MPL-2.0",
       "optional": true,
       "os": [