VPN: a WireGuard tunnel (#8)

The device joins a WireGuard network over whatever Wi-Fi it is on: one
peer, IPv4. A client's .conf is imported from the card (/vpn/wg0.conf) and
kept in the device's settings, private key included, never shown; Settings
offers to delete the file. A switch brings the tunnel up until the next
restart, "Start with Wi-Fi" every time; it waits for the clock, which a
handshake needs. VPN shows in the Status Bar.

The protocol is esphome/wireguard 0.4.8. It calls lwIP without lwIP's lock,
which this framework checks: every call into it is made with the lock held.

What goes through the tunnel is everything (AllowedIPs 0.0.0.0/0) or the
one subnet the device's tunnel address is in: lwIP routes by an
interface's subnet or by default, nothing finer. The import says how many
ranges it can't reach.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EhqxQ49eCju4CzKYNjZzwT

src/apps/vpn_page.h

@@ -0,0 +1,47 @@
+#pragma once
+
+#include <memory>
+#include <string>
+#include <vector>
+
+#include "dialog_model.h"
+#include "key_event.h"
+#include "key_help.h"
+#include "list_model.h"
+#include "services/clock_service.h"
+#include "services/storage_service.h"
+#include "services/vpn_service.h"
+#include "settings.h"
+#include "ui/canvas.h"
+
+namespace roro {
+
+// Settings > VPN (issue #8): the switch, "Start with Wi-Fi", importing a `.conf` from the card
+// and forgetting it, and what the tunnel is doing. No key is ever on this page.
+class VpnPage {
+   public:
+    static constexpr const char* kConfPath = "/vpn/wg0.conf";
+
+    VpnPage(Settings& settings, VpnService& vpn, StorageService& storage, ClockService& clock)
+        : settings_(settings), vpn_(vpn), storage_(storage), clock_(clock) {}
+
+    void enter();
+    bool onKey(const KeyEvent& e);  // false: leave the page
+    void draw(Canvas& c);
+    void help(std::vector<KeyHelp>& out) const;
+
+   private:
+    enum Row { kSwitch, kAuto, kImport, kForget, kRows };
+    enum class Ask { None, DeleteFile, Forget };
+
+    Settings& settings_;
+    VpnService& vpn_;
+    StorageService& storage_;
+    ClockService& clock_;
+    ListModel list_{kRows};
+    std::unique_ptr<DialogModel> confirm_;
+    Ask ask_ = Ask::None;
+    std::string message_;
+};
+
+}  // namespace roro