feat: render label badge pills using actual Gitea label hex colors

Update all label spans in issue/PR list and detail templates to use
background-color with the actual hex color from Gitea, replacing the
previous text-color-only styling. Add label-pill CSS class with text
shadow for readability against colored backgrounds.

Closes leeworks-agents/gitea-mobile#193

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

internal/templates/issue_detail.html

@@ -14,7 +14,7 @@
     </span>
     <span>{{.Issue.RepoOwner}}/{{.Issue.RepoName}} #{{.Issue.Number}}</span>
     {{range .Issue.Labels}}
-    <span class="label" style="color:#{{.Color}};border:1px solid #{{.Color}}">{{.Name}}</span>
+    <span class="label label-pill" style="background-color:#{{.Color}};color:#fff;border:1px solid #{{.Color}}">{{.Name}}</span>
     {{end}}
   </div>
   {{if .RenderedBody}}