fix: prevent silent config fallback and data loss on startup

This commit introduces fail-fast error handling for config loading failures,
replacing the previous silent fallback to default config which could cause
data loss (e.g., all user providers disappearing).

Key changes:

Backend (Rust):
- Replace AppState::new() with AppState::try_new() to explicitly propagate errors
- Remove Default trait to prevent accidental empty state creation
- Add init_status module as global error cache (OnceLock + RwLock)
- Implement dual-channel error notification:
  1. Event emission (low-latency, may race with frontend subscription)
  2. Command-based polling (reliable, guaranteed delivery)
- Remove unconditional save on startup to prevent overwriting corrupted config

Frontend (TypeScript):
- Add event listener for "configLoadError" (fast path)
- Add bootstrap-time polling via get_init_error command (reliable path)
- Display detailed error dialog with recovery instructions
- Prompt user to exit for manual repair

Impact:
- First-time users: No change (load() returns Ok(default) when file missing)
- Corrupted config: Application shows error and exits gracefully
- Prevents accidental config overwrite during initialization

Fixes the only critical issue identified in previous code review (silent
fallback causing data loss).

src-tauri/src/init_status.rs

@@ -0,0 +1,42 @@
+use serde::Serialize;
+use std::sync::{OnceLock, RwLock};
+
+#[derive(Debug, Clone, Serialize)]
+pub struct InitErrorPayload {
+    pub path: String,
+    pub error: String,
+}
+
+static INIT_ERROR: OnceLock<RwLock<Option<InitErrorPayload>>> = OnceLock::new();
+
+fn cell() -> &'static RwLock<Option<InitErrorPayload>> {
+    INIT_ERROR.get_or_init(|| RwLock::new(None))
+}
+
+pub fn set_init_error(payload: InitErrorPayload) {
+    if let Ok(mut guard) = cell().write() {
+        *guard = Some(payload);
+    }
+}
+
+pub fn get_init_error() -> Option<InitErrorPayload> {
+    cell().read().ok()?.clone()
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn init_error_roundtrip() {
+        let payload = InitErrorPayload {
+            path: "/tmp/config.json".into(),
+            error: "broken json".into(),
+        };
+        set_init_error(payload.clone());
+        let got = get_init_error().expect("should get payload back");
+        assert_eq!(got.path, payload.path);
+        assert_eq!(got.error, payload.error);
+    }
+}
+