feat: add missing import service and templates

These files were untracked and missing from prior commits, causing
ModuleNotFoundError on fresh deploys.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/templates/partials/import_form.html

@@ -0,0 +1,17 @@
+<article>
+    <header><h3>Import Old Database</h3></header>
+    <form hx-post="/dashboard/import" hx-encoding="multipart/form-data"
+          hx-target="#import-results" hx-swap="innerHTML">
+        <div class="grid">
+            <label>
+                Old Database File (.db)
+                <input type="file" name="db_file" accept=".db" required>
+            </label>
+            <label>
+                &nbsp;
+                <button type="submit">Import</button>
+            </label>
+        </div>
+    </form>
+    <div id="import-results"></div>
+</article>

app/templates/partials/import_results.html

@@ -0,0 +1,37 @@
+<table>
+    <thead>
+        <tr>
+            <th>Category</th>
+            <th>Imported</th>
+            <th>Skipped</th>
+        </tr>
+    </thead>
+    <tbody>
+        <tr>
+            <td>Workout Sessions</td>
+            <td>{{ result.sessions_imported }}</td>
+            <td>{{ result.sessions_skipped }}</td>
+        </tr>
+        <tr>
+            <td>Workout Logs</td>
+            <td>{{ result.logs_imported }}</td>
+            <td>{{ result.logs_skipped }}</td>
+        </tr>
+        <tr>
+            <td>Progress Log</td>
+            <td>{{ result.progress_imported }}</td>
+            <td>{{ result.progress_skipped }}</td>
+        </tr>
+    </tbody>
+</table>
+
+{% if result.warnings %}
+<details>
+    <summary>Warnings ({{ result.warnings|length }})</summary>
+    <ul>
+        {% for warning in result.warnings %}
+        <li><small>{{ warning }}</small></li>
+        {% endfor %}
+    </ul>
+</details>
+{% endif %}