Validate settings imports before writing secrets; disclose base URLs

A rejected import (bad env var name, disallowed host path) used to leave
keychain secrets already overwritten while the settings themselves stayed
unchanged. apply_settings_import now runs update_settings's validation
(extracted into validate_settings_update) before any secret write.

Also from this review round: sharpened two format-version tests that
previously passed against the pre-fix code too, added a direct test for
split_settings_and_secrets, warned on a dormant web terminal token even
when the terminal import leaves it off, matched the password-length check
to the frontend's unit of measure, zeroized the export plaintext buffer,
and surfaced non-blank Ollama/llama.cpp/OpenAI-compatible/gateway base
URLs in the import preview so a traffic redirect isn't silent.

app/src/lib/settingsImportPreview.ts

@@ -19,21 +19,37 @@ export function describeImport(preview: SettingsImportPreview): string[] {
   if (preview.has_gateway_api_key) items.push("The gateway provider API key");
   if (preview.has_gateway_master_key) items.push("The gateway master key");
   if (preview.has_web_terminal_access_token) items.push("The web terminal access token");
+  if (preview.ollama_base_url) items.push(`Ollama server: ${preview.ollama_base_url}`);
+  if (preview.llamacpp_base_url) items.push(`llama.cpp server: ${preview.llamacpp_base_url}`);
+  if (preview.openai_compatible_base_url) {
+    items.push(`OpenAI-compatible server: ${preview.openai_compatible_base_url}`);
+  }
+  if (preview.gateway_api_base) items.push(`Gateway upstream: ${preview.gateway_api_base}`);
   return items;
 }
 
 /**
  * Things about an import that deserve more attention than a bullet in a
- * long list — currently just the one, but deliberately its own function
- * rather than a flag inside `describeImport`: a setting that turns on a
- * network-listening service is exactly the kind of change a "your settings
- * were replaced" summary is bad at surfacing, on purpose or (if the file
- * came from someone else) not.
+ * long list — deliberately its own function rather than a flag inside
+ * `describeImport`: a setting that turns on a network-listening service is
+ * exactly the kind of change a "your settings were replaced" summary is bad
+ * at surfacing, on purpose or (if the file came from someone else) not.
+ *
+ * A token that arrives with the terminal left *off* gets its own warning
+ * too, distinct from the "enables it now" one: `start_web_terminal` only
+ * mints a fresh token when none is already set, so a planted token here
+ * would silently become live the next time someone flips the terminal on
+ * through the UI, with no import-time signal that it wasn't freshly
+ * generated.
  */
 export function describeImportWarnings(preview: SettingsImportPreview): string[] {
   const warnings: string[] = [];
   if (preview.enables_web_terminal) {
     warnings.push("Enables the remote web terminal, which listens on your network.");
+  } else if (preview.has_web_terminal_access_token) {
+    warnings.push(
+      "Includes a web terminal access token that will activate the next time the web terminal is turned on.",
+    );
   }
   return warnings;
 }