refactor(gui): split app.rs into models and panel modules

Extract types (models.rs), font settings, session controls, and send
panel into separate modules. Reduces app.rs from 1488 to 848 lines.
Zero behavior change.

- models.rs: ConnectionStatus, View, SendMode, LineEnding,
  DisplayOptions, SearchState, SessionTab
- panels/font_settings.rs: font selector and settings window
- panels/session_controls.rs: session connect/disconnect UI
- panels/send_panel.rs: send input and payload building

crates/pipeview-gui/src/models.rs

@@ -0,0 +1,133 @@
+use crate::buffers::{HexBuffer, PlotBuffer, TextBuffer};
+use crate::logging::LogWriter;
+use crate::panels::plot_view;
+use pipeview_client::config::SessionConfig;
+
+#[derive(Clone)]
+pub enum ConnectionStatus {
+    Connected,
+    Disconnected,
+    Connecting,
+    Error(String),
+}
+
+impl ConnectionStatus {
+    pub fn badge(&self) -> (&'static str, &'static str) {
+        match self {
+            Self::Connected => ("[connected]", "Connected"),
+            Self::Disconnected => ("[disconnected]", "Disconnected"),
+            Self::Connecting => ("[connecting]", "Connecting"),
+            Self::Error(_) => ("[error]", "Error"),
+        }
+    }
+}
+
+#[derive(Clone, Copy, PartialEq)]
+pub enum View {
+    Text,
+    Hex,
+    Plot,
+}
+
+#[derive(Clone, Copy, PartialEq)]
+pub enum SendMode {
+    Text,
+    Hex,
+}
+
+#[derive(Clone, Copy, PartialEq)]
+#[allow(clippy::upper_case_acronyms)]
+pub enum LineEnding {
+    None,
+    LF,
+    CR,
+    CRLF,
+}
+
+impl std::fmt::Display for LineEnding {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self {
+            Self::None => write!(f, "None"),
+            Self::LF => write!(f, "LF (\\n)"),
+            Self::CR => write!(f, "CR (\\r)"),
+            Self::CRLF => write!(f, "CRLF (\\r\\n)"),
+        }
+    }
+}
+
+#[derive(Clone, Copy)]
+pub struct DisplayOptions {
+    pub show_timestamp: bool,
+    pub show_direction: bool,
+    pub show_pipeline: bool,
+}
+
+#[derive(Clone, Default)]
+pub struct SearchState {
+    pub query: String,
+    pub matches: Vec<usize>,
+    pub current_match: usize,
+    pub case_sensitive: bool,
+    pub active: bool,
+    pub just_opened: bool,
+}
+
+impl SearchState {
+    pub fn clear(&mut self) {
+        self.query.clear();
+        self.matches.clear();
+        self.current_match = 0;
+        self.active = false;
+        self.just_opened = false;
+    }
+
+    pub fn next(&mut self) {
+        if !self.matches.is_empty() {
+            self.current_match = (self.current_match + 1) % self.matches.len();
+        }
+    }
+
+    pub fn prev(&mut self) {
+        if !self.matches.is_empty() {
+            self.current_match = if self.current_match == 0 {
+                self.matches.len() - 1
+            } else {
+                self.current_match - 1
+            };
+        }
+    }
+
+    pub fn match_count(&self) -> usize {
+        self.matches.len()
+    }
+
+    pub fn current_display(&self) -> usize {
+        if self.matches.is_empty() {
+            0
+        } else {
+            self.current_match + 1
+        }
+    }
+}
+
+pub struct SessionTab {
+    pub id: u64,
+    pub session_config: SessionConfig,
+    pub status: ConnectionStatus,
+    pub console: TextBuffer,
+    pub hex: HexBuffer,
+    pub plot: PlotBuffer,
+    pub plot_view: plot_view::PlotViewState,
+    pub view: View,
+    pub auto_reconnect: bool,
+    pub dtr: bool,
+    pub rts: bool,
+    pub send_input: String,
+    pub send_mode: SendMode,
+    pub line_ending: LineEnding,
+    pub send_status: Option<String>,
+    pub search: SearchState,
+    pub log_enabled: bool,
+    pub log_path: String,
+    pub log_writer: Option<LogWriter>,
+}