install.sh installs from your own server, not just from the forge

Milestone 325 step 5 (Scribe #3253).

    curl -fsSL https://notes.example.com/install.sh | sh

The server serves the installer at /install.sh with its own address written
into it (installer.py). Settings → Public address when set, the request's own
address otherwise. The substitution is one variable, given a value that has
passed a strict shape check, and the script checks it again; an address that
cannot pass makes the route refuse rather than serve a script pointed
elsewhere. `public_url` joins the live settings cache so the route needs no
database.

From a server, the script:
- resolves each Linux bundle from the public /api/client/<platform>, and
  builds the download URL from the platform id rather than reading it from
  the reply;
- asks for a device token (from the terminal, since stdin is the script),
  or takes TS_TOKEN, and sends it from a file rather than the command line;
- checks the sha256 the server published before anything installs;
- revokes a prompted token once the download is done;
- records `install-server` for the updater (step 6) instead of the channel.

The forge path is unchanged, and stays the default for the copy the forge
serves. The Account page's downloads card shows the one-line command
whenever the server holds a Linux client.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/src/components/ClientDownloads.vue

@@ -101,6 +101,13 @@ const missingPlatform = computed(() =>
   !lead.value.length && (family === "mac" || family === "ios") ? FAMILY_TITLE[family] : "",
 );
 
+// The one-command Linux install (milestone 325 step 5), shown wherever the server
+// holds a Linux client to install. Against the address this page was reached on,
+// which is the one the person can evidently reach; the script the server sends
+// back carries Settings → Public address when that is set.
+const hasLinux = computed(() => Object.keys(config.clients).some((id) => id.startsWith("linux-")));
+const installCommand = `curl -fsSL ${window.location.origin}/install.sh | sh`;
+
 // One decimal below 10 MB, none above: these sit in one list where a 2.7 MB
 // package and a 95 MB AppImage are compared, and "3 MB" next to "95 MB" loses the
 // only distinction that matters at the small end.
@@ -163,5 +170,18 @@ function readableSize(bytes: number): string {
         </li>
       </ul>
     </div>
+
+    <div v-if="hasLinux" class="mt-4">
+      <p class="text-xs text-neutral-400">
+        Or on Linux, from a terminal. It picks the right package for the system and asks
+        for a token from Linked devices to download it with.
+      </p>
+      <!-- select-all: one click takes the whole command, which is the only thing
+           anyone does with it. -->
+      <code
+        class="mt-1 block select-all break-all rounded bg-neutral-100 px-2 py-1 text-xs text-neutral-700 dark:bg-neutral-900 dark:text-neutral-200"
+        >{{ installCommand }}</code
+      >
+    </div>
   </section>
 </template>