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-esp32/src/tasks/render.rs

@@ -5,7 +5,7 @@ use client_domain::{
     BoundingBox, Color, DisplayPort, FontMetrics, RenderEngine, ScrollState, ThemeConfig,
 };
 use client_application::{ClientApp, RepaintCommand};
-use domain::{DisplayHint, Value};
+use domain::{DisplayHint, Value, WidgetError};
 use protocol::ServerMessage;
 use super::RenderEvent;
 use crate::config::RENDER_POLL_INTERVAL;
@@ -21,6 +21,7 @@ const COLOR_DISCONNECTED: Color = Color(200, 0, 0);
 struct WidgetCache {
     hint: DisplayHint,
     data: Vec<(String, Value)>,
+    error: Option<WidgetError>,
     bounds: BoundingBox,
     scroll: ScrollState,
 }
@@ -121,13 +122,15 @@ fn update_cache(engine: &RenderEngine, cmd: &RepaintCommand) -> WidgetCache {
         .iter()
         .map(|kv| (kv.key.clone(), kv.value.clone().into()))
         .collect();
+    let error: Option<WidgetError> = cmd.state.error.as_ref().map(|e| e.clone().into());
 
-    let content_h = engine.content_height(&hint, &data, cmd.bounds.width);
+    let content_h = engine.content_height(&hint, &data, cmd.bounds.width, error.as_ref());
     let scroll = ScrollState::new(cmd.bounds.height, content_h);
 
     WidgetCache {
         hint,
         data,
+        error,
         bounds: cmd.bounds,
         scroll,
     }
@@ -143,6 +146,7 @@ fn draw_widget(
         &cache.data,
         cache.bounds,
         cache.scroll.offset(),
+        cache.error.as_ref(),
     );
 
     for dc in &draw_cmds {