Let a spelling or grammar issue be dismissed

Not every flag deserves a fix, so give the checkers a way to be told no:

- ✖ beside an issue in the panel, and a matching entry in the editor's
  right-click menu, wave that issue away.
- A dismissal is remembered as the offending text paired with the message
  rather than a byte range, since offsets move as soon as you type. Both
  checkers filter their fresh results against it, so a dismissed complaint
  stays gone across re-checks and repeats of the same phrase in the file.
- The panel header counts the dismissals and ↩ takes them back. The offline
  spell check then rebuilds its own list; LanguageTool's can only come from
  the server, so those are dropped with a nudge towards ✓ Check.

Dismissals belong to the open file and the session — they are cleared when
another file or workspace is opened, and never written to disk. A name or
invented term still belongs in the word list, which persists.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011CVsxwa6YukFS2YFUY8W2j

src/app/workspace.rs

@@ -34,6 +34,7 @@ impl App {
         self.dirty = false;
         self.pending_delete = false;
         self.clear_lt();
+        self.dismissed.clear();
         self.session_start_counts = self.snapshot_counts();
         self.file_meta = self.snapshot_file_meta();
         self.rebuild_field_names();
@@ -365,6 +366,8 @@ impl App {
         self.spell_dirty = true;
         self.spell_last_edit = None;
         self.spell_menu = None;
+        // Dismissals are about this file's sentences, not the next one's.
+        self.dismissed.clear();
         let Some(name) = self.files.get(idx).cloned() else {
             return;
         };