feat: implement mobile-first HTMX templates and CSS

Build all frontend views using Go html/template with HTMX interactions
and mobile-first CSS with dark mode and iPhone safe areas.

Templates:
- layout.html: base layout with fixed bottom nav (Dashboard, Issues, PRs, Settings)
- dashboard.html: card-based triage view with priority indicators
- issues.html: issue list with filter bar (org, state) and infinite scroll
- pulls.html: PR list with diff stats and merge status
- issue_detail.html: issue detail with comments, label assignment
- pull_detail.html: PR detail with diff stats and review form
- create_issue.html: issue creation form with searchable repo selector

CSS (static/style.css):
- Mobile-first dark mode with CSS variables
- iPhone safe areas via env(safe-area-inset-bottom)
- Responsive: 2-column grid at 640px+ breakpoint
- Light mode support via prefers-color-scheme
- Card, label, badge, form, button component styles
- HTMX loading indicator and spinner animation
- Reduced motion support

HTMX patterns:
- hx-get with hx-target for SPA-like navigation
- hx-trigger="revealed" for infinite scroll
- hx-post with hx-swap for inline form actions
- Filter bar with hx-trigger="change" and hx-include

Closes leeworks-agents/gitea-mobile#5

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

internal/templates/create_issue.html

@@ -0,0 +1,42 @@
+{{define "content"}}
+<h1>Create Issue</h1>
+
+<form hx-post="/issues" hx-swap="innerHTML" hx-target="#main-content">
+  <div class="form-group">
+    <label for="repo-select">Repository</label>
+    <select id="repo-select" name="owner_repo" required>
+      <option value="">Select a repository...</option>
+      {{range $org, $repos := .Repos}}
+      <optgroup label="{{$org}}">
+        {{range $repos}}
+        <option value="{{.Owner.Login}}/{{.Name}}">{{.FullName}}</option>
+        {{end}}
+      </optgroup>
+      {{end}}
+    </select>
+    <input type="hidden" name="owner" id="owner-input">
+    <input type="hidden" name="repo" id="repo-input">
+  </div>
+
+  <div class="form-group">
+    <label for="title">Title</label>
+    <input type="text" id="title" name="title" placeholder="Issue title" required>
+  </div>
+
+  <div class="form-group">
+    <label for="body">Description</label>
+    <textarea id="body" name="body" placeholder="Describe the issue..."></textarea>
+  </div>
+
+  <button type="submit" class="btn btn-primary">Create Issue</button>
+</form>
+
+<script>
+  // Split owner/repo from select into hidden fields.
+  document.getElementById('repo-select').addEventListener('change', function() {
+    var parts = this.value.split('/');
+    document.getElementById('owner-input').value = parts[0] || '';
+    document.getElementById('repo-input').value = parts[1] || '';
+  });
+</script>
+{{end}}