feat(logging): persist diagnostics across restarts and redact secrets

Backend half of the logging overhaul.

Retention:
- Keep the last 4 rotated files at 20MB each and stop deleting logs on
  startup, so a crash's prior-run logs survive a restart.
- Apply the persisted log level right after plugin registration instead
  of at the end of setup.
- Bound crash.log with size-based rotation (5MB x 2).

Secret redaction on every log path:
- Proxy: strip userinfo/query from upstream URLs (keep path for
  diagnostics), exact-match redact known auth.api_key/access_token,
  classify request/response bodies instead of logging them, header
  allowlist.
- Redact the Gemini `?key=` in cache-trace endpoints.
- Omit MCP custom-field values (headers may carry tokens).
- Redact deeplink and model-fetch URLs before logging.

Docs:
- FAQ points users to the persistent crash.log for support workflows.

src-tauri/src/deeplink/provider.rs

@@ -122,7 +122,10 @@ pub fn import_provider_from_deeplink(
                 &provider_id,
                 normalized.clone(),
             ) {
-                log::warn!("Failed to add custom endpoint '{normalized}': {e}");
+                log::warn!(
+                    "Failed to add custom endpoint '{}': {e}",
+                    crate::url_for_log(&normalized)
+                );
             }
         }
     }