Make the feed URL editable, with a copy button

Entries and history are keyed by feed id, so changing a URL keeps them --
the point being that a feed URL can carry an auth token that gets rotated.
Changing it clears the stored ETag/Last-Modified, which belong to the old
URL and could otherwise produce a bogus 304.

The copy button cannot use navigator.clipboard: that needs a secure
context and this is served over plain HTTP on a LAN address. Falls back to
execCommand.

Invalid input now returns 400 rather than 500.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RPyeapneuXrCdojsaiXGbe

src/db.rs

@@ -231,6 +231,17 @@ impl Db {
         Ok(())
     }
 
+    /// Forgets the cached ETag/Last-Modified. Those validators belong to the old URL, so
+    /// keeping them across a URL change could produce a bogus 304 against the new one.
+    pub fn clear_validators(&self, feed_id: &str) -> Result<()> {
+        let conn = self.conn.lock().unwrap();
+        conn.execute(
+            "UPDATE feeds SET etag = NULL, last_modified = NULL, last_checked = NULL WHERE id = ?1",
+            [feed_id],
+        )?;
+        Ok(())
+    }
+
     pub fn set_feed_error(&self, feed_id: &str, url: &str, msg: &str) -> Result<()> {
         let conn = self.conn.lock().unwrap();
         conn.execute(