Upload an OPML file to import; tests for every way in and out

- The import screen has a file picker beside the paste box. The page
  reads the file, checks it looks like OPML before sending, and clears
  the picker when it is refused and after it is imported. The file is
  sent as text and never written to disk on the server.
- The server parses the OPML before touching anything and answers 400
  "that is not an OPML file" (was a 500). subscribe_opml takes a parsed
  document, so ipx import also refuses a non-OPML file by name.
- Tests: Settings' Export OPML download and paste import; uploading an
  RSS file (refused) and a real OPML; the server's 400; the admin's
  export round-tripped into a second account; ipx import/export in a
  scratch config.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016HdTEWQNrzyFULijigkmMn

src/main.rs

@@ -611,7 +611,9 @@ async fn import(ctx: &Ctx, config_path: &std::path::Path, file: &std::path::Path
         .into_iter()
         .find(|u| u.is_admin)
         .ok_or_else(|| anyhow::anyhow!("no admin account to subscribe: ipx user add <name> --admin"))?;
-    let (added, had) = subscribe_opml(ctx, config_path, &text, admin.id)?;
+    let doc = opml::OPML::from_str(&text)
+        .map_err(|e| anyhow::anyhow!("{} is not OPML: {e}", file.display()))?;
+    let (added, had) = subscribe_opml(ctx, config_path, &doc, admin.id)?;
     println!("subscribed {} to {added} feed(s); {had} already there", admin.name);
     Ok(())
 }
@@ -623,14 +625,15 @@ async fn import(ctx: &Ctx, config_path: &std::path::Path, file: &std::path::Path
 /// Before accounts, importing only added unknown URLs to config.toml. Once subscriptions
 /// decided what each person sees, that imported nothing at all for a feed someone else
 /// already had, and a new one had no subscriber, so it was never scanned.
+///
+/// The caller parses the document, so each refuses a file that is not OPML in its own terms,
+/// before anything is touched: a 400 from the web, a message from the CLI.
 pub fn subscribe_opml(
     ctx: &Ctx,
     config_path: &std::path::Path,
-    xml: &str,
+    doc: &opml::OPML,
     user_id: i64,
 ) -> Result<(usize, usize)> {
-    let doc = opml::OPML::from_str(xml)
-        .map_err(|e| anyhow::anyhow!("that does not parse as OPML: {e}"))?;
     let mut found = vec![];
     collect_outlines(&doc.body.outlines, &mut found);