feat(server): sous-commande idea --serve avec serveur HTTP sécurisé pairing+cookie (#13)

Lot B3 du chantier server/client mode : socle serveur sécurisé exposant le
cœur backend en mode client/serveur, sans PTY (reporté B5).

- crates/app-tauri/src/server.rs : sous-commande --serve, serveur HTTP
  sécurisé — /api/invoke sur allowlist, /api/pair → cookie de session,
  Origin strict, refus du secret en query, gate de bind. 16 tests dont les
  2 refus de sécurité (pairing_rejects_wrong_code,
  invoke_rejects_invalid_session_cookie).
- crates/app-tauri/src/lib.rs : câblage de la sous-commande --serve.
- crates/app-tauri/src/commands.rs : adaptations.
- crates/app-tauri/Cargo.toml : deps bytes/cookie/http/http-body-util,
  feature tokio net. Cargo.lock synchronisé.

Validé : cargo check --workspace vert, app-tauri 267 tests verts, cœur
backend agnostique confirmé, desktop non régressé.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

crates/app-tauri/Cargo.toml

@@ -26,14 +26,18 @@ infrastructure = { workspace = true }
 backend = { workspace = true }
 tauri = { workspace = true }
 tauri-plugin-dialog = { workspace = true }
-# `io-std` (on top of the workspace features) gives the headless `mcp-server`
-# bridge access to `tokio::io::{stdin,stdout}` without widening tokio elsewhere.
-tokio = { workspace = true, features = ["io-std", "rt"] }
+# `io-std` gives the headless `mcp-server` bridge access to
+# `tokio::io::{stdin,stdout}`; `net` is used only by the `--serve` HTTP adapter.
+tokio = { workspace = true, features = ["io-std", "rt", "net"] }
 serde = { workspace = true }
 serde_json = { workspace = true }
 thiserror = { workspace = true }
 uuid = { workspace = true }
 base64 = "0.22"
+bytes = "1.11"
+cookie = "0.18"
+http = "1.4"
+http-body-util = "0.1"
 # `AppAgentResumer` implements the application's async `AgentResumer` port (LS7).
 async-trait = { workspace = true }
 # Cross-OS local IPC for the MCP loopback transport (M5a): Unix domain socket