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/issues.html

@@ -5,7 +5,7 @@
     <div class="card-meta">
       <span>{{.RepoOwner}}/{{.RepoName}} #{{.Number}}</span>
       {{range .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}}
       {{if .Assignee}}
       <img src="{{.Assignee.AvatarURL}}" alt="{{.Assignee.Login}}" class="avatar" title="Assigned to {{.Assignee.Login}}">