Basic frontend

I'm sorry for the big commit

- Added yarte templating engine
- Wrote frontend: all HTML except file input area, basic CSS (themes soon), JS.
No client-side encryption yet. And no working API.
- Simple but handy build system in Makefile
(requires pacman -S entr / apt install entr)

scss/style.scss

@@ -0,0 +1,66 @@
+body {
+  margin: 0;
+  padding: 0;
+}
+
+main {
+  max-width: 50rem;
+  margin: 0 auto;
+  padding: 0 0.5rem;
+
+  font-family: system-ui, sans-serif;
+}
+
+header {
+  display: flex;
+  flex-direction: row;
+  justify-content: center;
+  align-items: center;
+  column-gap: 1.25rem;
+  font-size: 1.125rem;
+}
+
+header>label {
+  cursor: pointer;
+}
+
+.htab {
+  display: none;
+}
+
+#htab-file:checked~#tab-link {
+  display: none;
+}
+
+#htab-link:checked~#tab-file {
+  display: none;
+}
+
+.form-footer {
+  display: flex;
+  flex-direction: row;
+  justify-content: space-between;
+
+  div {
+    display: flex;
+    flex-direction: column;
+  }
+
+  div:last-child {
+    justify-content: end;
+  }
+
+  label {
+    display: flex;
+    flex-direction: row;
+    align-items: center;
+    gap: 0.25rem;
+  }
+
+  input[type=radio],
+  input[type=checkbox] {
+    margin: 0;
+    margin-left: 0.25rem;
+    padding: 0;
+  }
+}