fix: legible caret and working :q!

CodeMirror injects its own `.cm-cursor` rule after our stylesheet, so the
equally specific override lost and the caret painted black on a dark
background. Qualify the selector, same as `.cm-activeLine`.

`:q!` reached the command table as the literal name `quit!`, which nothing
matched. Commands now carry an optional `bang` variant, and `runCommand`
splits a trailing `!` off any head before looking it up.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01476ZKdB4k3rPDwvVhXcnDg

web/src/lib/Help.svelte

@@ -46,6 +46,10 @@
             {#if spec.args}<span class="faint"> {spec.args}</span>{/if}
           </dt>
           <dd>{spec.help}</dd>
+          {#if spec.bang}
+            <dt>:{spec.name}!{#if spec.aliases?.length}<span class="faint"> ({spec.aliases.map((a) => `${a}!`).join(' ')})</span>{/if}</dt>
+            <dd>{spec.bang.help}</dd>
+          {/if}
         {/each}
       </dl>
     </section>