Add native self-hosted instance connection to fluxer_desktop

Trimmed monorepo checkout (fluxer_desktop + packages/voice_engine_v2 +
tools/ci) with a "Connect to a Different Server" menu item and popout
that lets the desktop app switch to any self-hosted Fluxer instance,
plus fixes for well-known discovery on single-domain self-hosted
deployments and a false-positive ERR_ABORTED on same-origin client
redirects during the switch. Defaults to chat.fluxr.chat and uses an
isolated userData directory from the official build.

fluxer_desktop/native/linux-portals/src/portal.rs

@@ -0,0 +1,50 @@
+// SPDX-License-Identifier: AGPL-3.0-or-later
+
+use std::sync::atomic::{AtomicU64, Ordering};
+use std::time::{SystemTime, UNIX_EPOCH};
+
+pub const REQUEST_INTERFACE: &str = "org.freedesktop.portal.Request";
+
+pub fn request_path(unique_bus_name: &str, handle_token: &str) -> String {
+    let trimmed = unique_bus_name.strip_prefix(':').unwrap_or(unique_bus_name);
+    let mut out = String::with_capacity(40 + trimmed.len() + handle_token.len());
+    out.push_str("/org/freedesktop/portal/desktop/request/");
+    for ch in trimmed.chars() {
+        out.push(if ch == '.' { '_' } else { ch });
+    }
+    out.push('/');
+    out.push_str(handle_token);
+    out
+}
+
+static TOKEN_SEQ: AtomicU64 = AtomicU64::new(1);
+
+pub fn mint_token(prefix: &str) -> String {
+    let seq = TOKEN_SEQ.fetch_add(1, Ordering::Relaxed);
+    let ms = SystemTime::now()
+        .duration_since(UNIX_EPOCH)
+        .map(|d| d.as_millis() as u64)
+        .unwrap_or(0);
+    format!("{prefix}_{ms:x}_{seq:x}")
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn request_path_sanitizes_unique_bus_name() {
+        assert_eq!(
+            request_path(":1.42", "fluxer_fc_open_1"),
+            "/org/freedesktop/portal/desktop/request/1_42/fluxer_fc_open_1"
+        );
+    }
+
+    #[test]
+    fn mint_token_is_distinct_and_prefixed() {
+        let a = mint_token("fluxer_fc_open");
+        let b = mint_token("fluxer_fc_open");
+        assert_ne!(a, b);
+        assert!(a.starts_with("fluxer_fc_open_"));
+    }
+}