state recovery, polling optimizations, error rendering

widget states cached to SQLite, loaded on startup to seed DataProjection
so server restart preserves last-known data for reconnecting clients.

polling: first poll runs immediately, widget list cached per-task with
30s refresh, static text polled once inline instead of looping.

poll failures propagate WidgetError::SourceUnavailable to clients.
render engine prepends [offline] prefix in accent color, stale data
preserved below.

crates/client-domain/src/render_engine.rs

@@ -2,7 +2,7 @@ use crate::{
     BoundingBox, Color, FontMetrics, FontSize, ThemeConfig, alignment::align_offset,
     markup::parse_markup, text_layout::wrap_lines,
 };
-use domain::{DisplayHint, DisplayHintKind, HAlign, VAlign, Value};
+use domain::{DisplayHint, DisplayHintKind, HAlign, VAlign, Value, WidgetError};
 
 #[derive(Debug, Clone, PartialEq)]
 pub struct DrawCommand {
@@ -92,8 +92,12 @@ impl RenderEngine {
         data: &[(String, Value)],
         bounds: BoundingBox,
         scroll_offset: u16,
+        error: Option<&WidgetError>,
     ) -> Vec<DrawCommand> {
-        let text = self.format_widget(hint, data);
+        let mut text = self.format_widget(hint, data);
+        if error.is_some() {
+            text = format!("{{accent}}[offline]{{/}}\n{text}");
+        }
         let mut cmds = self.render_text(&text, bounds, hint.h_align, hint.v_align);
 
         if scroll_offset > 0 {
@@ -110,8 +114,17 @@ impl RenderEngine {
         cmds
     }
 
-    pub fn content_height(&self, hint: &DisplayHint, data: &[(String, Value)], width: u16) -> u16 {
-        let text = self.format_widget(hint, data);
+    pub fn content_height(
+        &self,
+        hint: &DisplayHint,
+        data: &[(String, Value)],
+        width: u16,
+        error: Option<&WidgetError>,
+    ) -> u16 {
+        let mut text = self.format_widget(hint, data);
+        if error.is_some() {
+            text = format!("{{accent}}[offline]{{/}}\n{text}");
+        }
         let plain: String = parse_markup(&text, &self.theme)
             .iter()
             .map(|s| s.text.as_str())