feat(webui): consolidate the page scripts into a single main.js bundle

All 17 page scripts (i18n, shared helpers, navigation and the per-page
scripts) are merged into one html/main.js that every page loads as its
only script, so the browser fetches it once and gzip can compress
across all of it.

The page-specific initialisation is guarded by the presence of the
matching page elements (it used to be selected implicitly by which
script a page loaded); the sidebar fill and the stat popup wiring moved
into DOMContentLoaded handlers because the bundle loads in <head>,
before the body exists.

tools/httpd_sim.h: BUFFER_SIZE 24000 -> 131072 so the simulator can
serve the larger bundle (the firmware is unaffected, it serves the
gzip'd bundle of ~17 KB).

Measured (make MACHINE=KP_9000_6XHML_X2): the JS drops from 23,302 to
16,948 bytes gzip; the whole Web UI from 34,784 to 28,200 bytes (25.3 %
of the raw 111,280); the data block ends at 0x46e28.  Verified with a
headless browser against httpd_sim and against a real PCB-K0402WS-V3:
all pages render, the sidebar and the port strip are drawn, no
JavaScript errors.

html/mirror.html

@@ -1,7 +1,6 @@
 <!DOCTYPE html>
 <html>
   <script src="/main.js"></script>
-  <script src="/i18n.js"></script>
   <link rel="stylesheet" href="style.css">
   <title data-i18n="mirror_title">Mirror Configuration</title>
   </head>
@@ -19,9 +18,6 @@
     <div id="mPortsRX"></div>
     <br/> <input style="width:15%;" class="action" id="mirror_sub" onclick="mirrorSub();" type="button" data-i18n="mirror_update" value="Update / Create">
     <input style="width:15%;" class="action" id="mirror_del" onclick="mirrorDel();" type="button" data-i18n="mirror_disable" value="Disable Mirroring">
-    <script src="/mirror.js"></script>
-    <script src="/mirror_sub.js"></script>
     </div>
-    <script src="/navigation.js"></script>
   </body>
 </html>