feat(webui): align dashboard with re:cinq brand guidelines v1.0

[nextlevelshit]

Summary

• Replace all CSS color variables with re:cinq brand palette (Midnight Navy, Quantum Blue, Pulse Blue, Soft Indigo, Aurora gradient)
• Update dark mode backgrounds from GitHub-dark (#0d1117) to Midnight Navy (#0F1F49)
• Update light mode backgrounds to Crystal White / Neutral Fog, buttons and links to Quantum Blue (#0014EB)
• Add Neue Montreal font via CDN with sensible sans-serif fallbacks
• Replace inline Wave SVG logo with re:cinq leaf logomark in navbar

Related to #684

Changes

• internal/webui/static/style.css — replaced all colour custom properties in :root and [data-theme="light"] blocks with brand palette hex values; added Neue Montreal @import; updated button, card, table, and status styles for brand consistency
• internal/webui/templates/layout.html — replaced inline Wave SVG with re:cinq leaf logomark SVG; updated navbar brand text
• docs/public/logo.svg — updated logo SVG to re:cinq leaf logomark
• specs/684-webui-brand-guidelines/ — spec, plan, and tasks documents for the change

Test Plan

• Ran wave serve and verified dashboard loads cleanly in browser
• Confirmed dark mode uses Midnight Navy backgrounds
• Confirmed light mode uses Crystal White / Neutral Fog
• Verified Neue Montreal font renders with fallback
• Verified navbar displays re:cinq leaf logomark
• Checked status colours (completed/running/failed) remain distinguishable against new backgrounds
• No legacy colour values (#4f46e5, #0d1117, #818cf8, #8b5cf6) remain in CSS