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/pull_detail.html

@@ -0,0 +1,47 @@
+{{define "content"}}
+<h1>{{.Pull.Title}}</h1>
+
+<div class="card">
+  <div class="card-meta">
+    <span class="type-badge type-pull">PR</span>
+    <span class="state-open">{{.Pull.State}}</span>
+    <span>{{.Pull.RepoOwner}}/{{.Pull.RepoName}} #{{.Pull.Number}}</span>
+    {{range .Pull.Labels}}
+    <span class="label" style="color:#{{.Color}};border:1px solid #{{.Color}}">{{.Name}}</span>
+    {{end}}
+  </div>
+  <div class="card-meta" style="margin-top:0.5rem;">
+    <span class="diff-add">+{{.Pull.Additions}}</span>
+    <span class="diff-del">-{{.Pull.Deletions}}</span>
+    {{if .Pull.Mergeable}}<span style="color:var(--accent-green);">Mergeable</span>{{end}}
+  </div>
+  {{if .Pull.Body}}
+  <div class="card-body">{{.Pull.Body}}</div>
+  {{end}}
+</div>
+
+<div class="card" style="margin-top:1rem;">
+  <h2>Submit Review</h2>
+  <form hx-post="/pulls/{{.Pull.RepoOwner}}/{{.Pull.RepoName}}/{{.Pull.Number}}/review" hx-swap="outerHTML">
+    <div class="form-group">
+      <label for="review-body">Comment</label>
+      <textarea id="review-body" name="body" placeholder="Leave a review comment..."></textarea>
+    </div>
+    <div class="review-options">
+      <label class="review-option">
+        <input type="radio" name="event" value="COMMENT" checked>
+        <span>Comment</span>
+      </label>
+      <label class="review-option">
+        <input type="radio" name="event" value="APPROVED">
+        <span>Approve</span>
+      </label>
+      <label class="review-option">
+        <input type="radio" name="event" value="REQUEST_CHANGES">
+        <span>Request Changes</span>
+      </label>
+    </div>
+    <button type="submit" class="btn btn-primary">Submit Review</button>
+  </form>
+</div>
+{{end}}