Use Dylan's DM logo, switch accent to brand blue, add dark mode

Recreate the DM monogram and wordmark as SVG and type so they stay sharp in
both themes, move the accent from violet to the logo's blue, and add a small
theme toggle in the nav that follows the system until the visitor picks one.

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

static/favicon.svg

@@ -1 +1 @@
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+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 64 64"><rect width="64" height="64" rx="16" fill="#0b0b0d"/><g transform="translate(6 15) scale(.5)"><path fill="#fff" fill-rule="evenodd" d="M9 4h55c10 0 13 7 9 14L58 46c-3 6-8 8-15 8.5L3 57zM21.5 15.5 18.3 45.8 40 44.5c4-.3 6-1.5 7.5-4.3l12-21.4c1.2-2.2.3-3.3-2-3.3z"/><path fill="#33b9ff" d="M24 59 102 1 91 52 72 54.5 79 33z"/><path fill="#8fdcff" d="M24 59 102 1 79 33z"/></g></svg>