Serve the BIMI logo, favicon.ico and an apple-touch icon as real files

ultimo.email is the webmail itself, so any path that was not a real file fell
back to the app's index.html: /bimi/ultimo-logo.svg (the BIMI record's logo)
and /favicon.ico both came back as the webmail page. They now ship in
frontend/public, which nginx serves before the SPA fallback. The BIMI SVG
moves there from frontend/icons, so there is one copy.

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

frontend/public/bimi/ultimo-logo.svg

@@ -0,0 +1,14 @@
+<svg xmlns="http://www.w3.org/2000/svg" version="1.2" baseProfile="tiny-ps" viewBox="0 0 512 512">
+<title>Ultimo Email</title>
+<desc>Ultimo Email logo, for BIMI (SVG Tiny Portable/Secure)</desc>
+<defs>
+<linearGradient id="g" gradientUnits="userSpaceOnUse" x1="44.1352" y1="192.6008" x2="44.1352" y2="224.5123">
+<stop offset="0.202" stop-color="#f89629"/>
+<stop offset="1" stop-color="#f74176"/>
+</linearGradient>
+</defs>
+<rect width="512" height="512" fill="#ffffff"/>
+<g transform="translate(97.2800,104.2299) scale(7.063024) translate(-21.70051,-187.41349)">
+<path fill="url(#g)" d="m 58.875622,191.58757 c -2.152336,-0.00074 -4.067804,1.84724 -4.067104,4.1266 v 2.85212 c -0.073,-0.005 -0.146556,-0.007 -0.220195,-0.007 H 44.088203 33.589648 c -0.04176,-0.00007 -0.0835,0.00059 -0.125155,0.002 v -2.78918 c -0.000166,-2.27807 -1.959264,-4.12477 -4.110377,-4.12494 -2.151113,0.00018 -4.208743,1.84687 -4.208908,4.12494 v 15.55127 c -0.000039,0.005 -0.02954,3.84339 2.173004,7.60498 2.20396,3.76402 7.038201,7.38257 14.452668,7.38257 h 4.580673 c 7.417703,0 12.26918,-3.66617 14.505859,-7.44221 2.23668,-3.77604 2.27039,-7.63481 2.27039,-7.63481 l 0.0016,-0.0344 V 195.7141 c 0.0007,-2.27872 -2.102019,-4.12642 -4.253744,-4.1266 z m -2.007917,7.94794 v 11.57584 0.0286 c 0,0 -0.06644,2.0824 -1.311391,4.18416 -0.572022,0.96572 -1.445415,2.15762 -2.917676,2.99793 -1.472263,0.8403 -3.460652,1.33079 -6.272222,1.33079 H 41.78535 c -2.82381,0 -4.798635,-0.47938 -6.256969,-1.30429 -1.458334,-0.8249 -2.313565,-2.00343 -2.85901,-2.93496 -1.196636,-2.04365 -1.230039,-4.07193 -1.230039,-4.07193 l -0.0011,-0.0141 v -0.0157 -11.69637 l 0.02191,0.0178 0.000782,0.001 c 0.03515,0.0487 0.07994,0.0958 0.133761,0.14041 l 5.513859,4.57144 5.515036,4.57144 c 0.651946,0.53945 2.279028,0.53945 2.930974,0 l 5.515034,-4.57144 5.514252,-4.57144 c 0.01986,-0.0164 0.03852,-0.0331 0.05593,-0.0501 l 0.000782,-0.00082 z"/>
+</g>
+</svg>