tui: dedicated MOTD renderer (M7-5)

The MOTD used to ride on tui_render_command_output's coattails — text
got stuffed into client->command_output prefixed by "=== 公告 / MOTD ==="
and rendered under a reverse-video " COMMAND OUTPUT " title bar.  Two
different things (a transient command result vs. a one-shot service
notice) wearing the same chrome.

Now MOTD has its own renderer with its own aesthetic:

    ╭─ 公告 / MOTD ────────────────────────────────╮

      欢迎来到 TNT 公共聊天室。
      请互相尊重，不要刷屏。
      管理员：m1ng

    ╰─ 按任意键继续 / press any key ────────────────╯

- title chip embedded in the top border (bright cyan)
- footer hint embedded in the bottom border (dim grey)
- 2-column left padding on body lines, blank top/bottom pad rows
- dim cyan borders, no full-line reverse anywhere

Wiring:
- new tui_render_motd() declared in tui.h
- new client_t.show_motd flag selects the renderer; command_output
  remains the text storage (no extra buffer needed)
- input.c MOTD path sets show_motd = true and calls tui_render_motd()
- handle_key's dismiss path clears show_motd alongside command_output
- main-loop redraw dispatch checks show_motd before command_output[0]

include/ssh_server.h

@@ -25,6 +25,7 @@ typedef struct client {
     int command_history_count;
     int command_history_pos;
     char command_output[2048];
+    bool show_motd;                  /* command_output holds MOTD text */
     char exec_command[MAX_EXEC_COMMAND_LEN];
     char ssh_login[MAX_USERNAME_LEN];
     time_t connect_time;

include/tui.h

@@ -16,6 +16,11 @@ void tui_render_help(struct client *client);
 /* Render the command output screen */
 void tui_render_command_output(struct client *client);
 
+/* Render the MOTD screen.  Reads the message text from
+ * client->command_output (shared storage); the show_motd flag selects
+ * this renderer over tui_render_command_output. */
+void tui_render_motd(struct client *client);
+
 /* Render the input line */
 void tui_render_input(struct client *client, const char *input);
 

src/input.c

@@ -194,9 +194,10 @@ static bool handle_key(client_t *client, unsigned char key, char *input) {
         return true;  /* Key consumed */
     }
 
-    /* Handle command output display */
+    /* Handle command output / MOTD display: any key dismisses */
     if (client->command_output[0] != '\0') {
         client->command_output[0] = '\0';
+        client->show_motd = false;
         client->mode = MODE_NORMAL;
         tui_render_screen(client);
         return true;  /* Key consumed */
@@ -447,9 +448,11 @@ void input_run_session(client_t *client) {
                 fclose(motd_fp);
                 if (motd_len > 0) {
                     motd_buf[motd_len] = '\0';
-                    snprintf(client->command_output, sizeof(client->command_output),
-                             "=== 公告 / MOTD ===\n%s", motd_buf);
-                    tui_render_command_output(client);
+                    snprintf(client->command_output,
+                             sizeof(client->command_output),
+                             "%s", motd_buf);
+                    client->show_motd = true;
+                    tui_render_motd(client);
                     seen_update_seq = room_get_update_seq(g_room);
                     goto main_loop;
                 }
@@ -491,6 +494,8 @@ main_loop:
 
                 if (client->show_help) {
                     tui_render_help(client);
+                } else if (client->show_motd) {
+                    tui_render_motd(client);
                 } else if (client->command_output[0] != '\0') {
                     tui_render_command_output(client);
                 } else {

src/tui.c

@@ -543,7 +543,95 @@ void tui_render_command_output(client_t *client) {
     client_send(client, buffer, pos);
 }
 
-/* Get help text based on language */
+/* Render the MOTD screen.
+ *
+ * A framed banner with a title chip embedded in the top border and an
+ * "any key to continue" hint embedded in the bottom border, MOTD body
+ * left-padded inside.  Dismissed by handle_key like any other modal
+ * (sets command_output[0]='\0' and show_motd=false).
+ *
+ * Lighter aesthetic than tui_render_command_output: no full-line reverse,
+ * dim borders, two blank lines of breathing room above and below the
+ * body so the announcement reads as a notice rather than a console dump. */
+void tui_render_motd(client_t *client) {
+    if (!client || !client->connected) return;
+
+    int rw = client->width;
+    int rh = client->height;
+    if (rw < 10) rw = 10;
+    if (rh < 4) rh = 4;
+
+    char buffer[4096];
+    size_t pos = 0;
+    buffer_appendf(buffer, sizeof(buffer), &pos, ANSI_CLEAR ANSI_HOME);
+
+    /* Top border: ╭─ 公告 / MOTD ──...──╮ */
+    const char *title = " 公告 / MOTD ";
+    int title_w = utf8_string_width(title);
+    int top_dash_fill = rw - 2 - title_w - 1;  /* 2 corners, 1 leading ─ */
+    if (top_dash_fill < 0) top_dash_fill = 0;
+
+    buffer_appendf(buffer, sizeof(buffer), &pos, "\033[2;36m╭─");
+    buffer_appendf(buffer, sizeof(buffer), &pos, "\033[0;1;36m%s\033[2;36m", title);
+    for (int i = 0; i < top_dash_fill; i++) {
+        buffer_append_bytes(buffer, sizeof(buffer), &pos, "─", strlen("─"));
+    }
+    buffer_appendf(buffer, sizeof(buffer), &pos, "╮\033[0m\r\n");
+
+    /* Top breathing-room line */
+    buffer_appendf(buffer, sizeof(buffer), &pos, "\r\n");
+
+    /* Body lines (left-pad 2 cols, truncate to inner width) */
+    char body_copy[2048];
+    strncpy(body_copy, client->command_output, sizeof(body_copy) - 1);
+    body_copy[sizeof(body_copy) - 1] = '\0';
+
+    int body_lines = 0;
+    int max_body_lines = rh - 4;  /* top border + top pad + bottom pad + bottom border */
+    if (max_body_lines < 1) max_body_lines = 1;
+
+    char *line = strtok(body_copy, "\n");
+    while (line && body_lines < max_body_lines) {
+        char truncated[1024];
+        strncpy(truncated, line, sizeof(truncated) - 1);
+        truncated[sizeof(truncated) - 1] = '\0';
+
+        int avail = rw - 4;  /* 2 cols padding each side */
+        if (avail < 4) avail = 4;
+        if (utf8_string_width(truncated) > avail) {
+            utf8_truncate(truncated, avail);
+        }
+        buffer_appendf(buffer, sizeof(buffer), &pos, "  %s\r\n", truncated);
+        body_lines++;
+        line = strtok(NULL, "\n");
+    }
+
+    /* Fill empty space up to the bottom border */
+    int used_rows = 1 /*top*/ + 1 /*pad*/ + body_lines + 1 /*pad*/ + 1 /*bottom*/;
+    int filler_rows = rh - used_rows;
+    if (filler_rows < 0) filler_rows = 0;
+    for (int i = 0; i < filler_rows; i++) {
+        buffer_appendf(buffer, sizeof(buffer), &pos, "\r\n");
+    }
+
+    /* Bottom breathing-room line */
+    buffer_appendf(buffer, sizeof(buffer), &pos, "\r\n");
+
+    /* Bottom border: ╰─ 按任意键继续 ─...─╯ */
+    const char *footer = " 按任意键继续 / press any key ";
+    int footer_w = utf8_string_width(footer);
+    int bot_dash_fill = rw - 2 - footer_w - 1;
+    if (bot_dash_fill < 0) bot_dash_fill = 0;
+
+    buffer_appendf(buffer, sizeof(buffer), &pos, "\033[2;36m╰─");
+    buffer_appendf(buffer, sizeof(buffer), &pos, "\033[0;2;37m%s\033[2;36m", footer);
+    for (int i = 0; i < bot_dash_fill; i++) {
+        buffer_append_bytes(buffer, sizeof(buffer), &pos, "─", strlen("─"));
+    }
+    buffer_appendf(buffer, sizeof(buffer), &pos, "╯\033[0m");
+
+    client_send(client, buffer, pos);
+}
 const char* tui_get_help_text(help_lang_t lang) {
     if (lang == LANG_EN) {
         return "TERMINAL CHAT ROOM - HELP\n"