fix: limit the prompt template to 1,000 characters

storage.sync rejects any item over 8192 bytes, and the options page
writes every field in one set(). An oversized template therefore made
every later save fail, including changes to unrelated settings, with
only a brief "Could not save settings".

Cap the template at 1,000 UTF-16 units: maxlength on the input, and
mergeSettings falls back to the default for a longer stored value, as it
does for other out-of-range settings. Even a template of pasted control
characters (six bytes each once JSON-escaped) stays under the quota; a
test pins that arithmetic and another ties the HTML maxlength to the
constant.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

@@ -74,6 +74,7 @@ Open `about:addons`, select this extension, and choose the **Options** tab
   guarantee.
 - **Open in** — a new tab, a new background tab, or a new window.
 - **Prompt template** — wraps your text, for example `Explain simply: {query}`.
+  Up to 1,000 characters.
 
 Restore defaults resets extension preferences to Simple mode with translation
 hidden; it does not change Firefox's search engine or shortcut settings.