ui: redesign brand icon and fix clipped wordmark

- new mark: measurement-span glyph on rounded brand-gradient badge
  (favicon, inline logo, standalone tmflow-logo.svg)
- wordmark moved from SVG <text> with hardcoded x offsets (clipped
  "Flow" past the 260px viewBox, font-dependent overlap) to plain HTML
  spans — never clips, natural kerning, theme-adaptive
- _app_logo.html params: logo_class, wordmark_class, logo_id (unique
  gradient id when logo appears twice per page)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/frontend/flask_app/templates/auth/login.html

@@ -16,7 +16,9 @@
       <!-- Logo -->
       <div class="text-center mb-8">
         <div class="mx-auto mb-4 inline-flex justify-center text-slate-900 dark:text-white">
-          {% set logo_class = 'h-16 w-auto' %}
+          {% set logo_class = 'h-14 w-14' %}
+          {% set wordmark_class = 'text-3xl' %}
+          {% set logo_id = 'login' %}
           {% include 'components/_app_logo.html' %}
         </div>
         <p class="text-sm text-[var(--text-secondary)]">