FIX: use correct iframe selector and defer init until Docs is ready

[bennidhamma]

Problem

The extension shows the NORMAL mode indicator but keystrokes are not intercepted — text inserts as if in insert mode.

Two bugs cause this:

1. Wrong iframe: content.js used getElementsByTagName('iframe')[0], which returns the first iframe in DOM order. Google Docs has multiple iframes; the text event target is .docs-texteventtarget-iframe. The keydown listener was attached to the wrong element, so keystrokes were never intercepted. (page_script.js already used the correct selector — this brings content.js in line with it.)

2. Timing race: Google Docs injects its iframes dynamically after document_idle. Even with the correct selector, the element may not exist when the content script runs. A MutationObserver now defers initialization until the target iframe appears. cursorTop (.kix-cursor-top) is fetched lazily for the same reason.

Changes

• Replace getElementsByTagName('iframe')[0] with document.querySelector('.docs-texteventtarget-iframe')
• Wrap initialization in a function; if the element isn't present yet, a MutationObserver retries until it is
• Fetch cursorTop lazily in switchModeToNormal / switchModeToInsert with a null guard

Test plan

• Load a Google Doc — NORMAL mode indicator appears and vim motions work immediately
• Hard-refresh the doc — extension still initializes correctly
• Press i to enter insert mode, type text, press Escape — returns to NORMAL and keystrokes are intercepted again