feat: add org filter dropdown to Dashboard view

Add an org filter select to the dashboard that allows users to narrow
the triage queue to a specific organization. The filter uses HTMX to
update the view without a full page reload, mirroring the pattern
already used in the issues and pulls views.

Closes leeworks-agents/gitea-mobile#68

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

internal/templates/dashboard.html

@@ -1,6 +1,17 @@
 {{define "content"}}
 <h1>Dashboard</h1>
 
+{{if gt (len .Orgs) 1}}
+<div class="filter-bar">
+  <select name="org" hx-get="/" hx-trigger="change" hx-target="#main-content" hx-swap="innerHTML" hx-push-url="true">
+    <option value="">All orgs</option>
+    {{range .Orgs}}
+    <option value="{{.}}" {{if eq . $.SelectedOrg}}selected{{end}}>{{.}}</option>
+    {{end}}
+  </select>
+</div>
+{{end}}
+
 {{if .Error}}
 <p class="empty">{{.Error}}</p>
 {{else if not .Items}}