feat(accounts): implement user accounts, sessions, and dashboards

Introduce Stage 4 features to support multi-user accounts, cookie-based web sessions, and personal dashboards.

Changes include:
- Adding `/register` to bootstrap the first admin account and `/login`/`/logout` for session management.
- Creating a personal dashboard (`/app`) to display owned boxes, storage usage, and upload history.
- Implementing admin user management (`/admin/users`) for generating invite links and managing user states.
- Updating the bbolt database schema to store users, sessions, invites, and collections.
- Adding `golang.org/x/crypto` for password hashing and introducing unit tests for account handlers.

backend/static/css/app.css

@@ -90,12 +90,17 @@ svg {
 
 .brand,
 .nav-links,
-.footer-links {
+.footer-links,
+.inline-form {
   display: inline-flex;
   align-items: center;
   gap: 0.5rem;
 }
 
+.inline-form {
+  margin: 0;
+}
+
 .brand {
   font-weight: 650;
   text-decoration: none;
@@ -171,6 +176,125 @@ h1 {
   padding: 1.5rem;
 }
 
+.auth-view {
+  width: min(28rem, calc(100% - 2rem));
+  min-height: calc(100vh - 7.25rem);
+  margin: 0 auto;
+  padding: 3rem 0;
+  display: grid;
+  place-items: center;
+}
+
+.auth-card {
+  box-shadow: var(--shadow);
+}
+
+.kicker {
+  margin: 0 0 0.5rem;
+  color: var(--muted-foreground);
+  font-size: 0.76rem;
+  font-weight: 700;
+  letter-spacing: 0;
+  text-transform: uppercase;
+}
+
+.muted-copy,
+.auth-alt {
+  color: var(--muted-foreground);
+  font-size: 0.9rem;
+  line-height: 1.5;
+}
+
+.stack-form {
+  display: grid;
+  gap: 0.9rem;
+  margin-top: 1rem;
+}
+
+.stack-form label,
+.inline-controls label,
+.collection-create label {
+  display: grid;
+  gap: 0.35rem;
+  color: var(--muted-foreground);
+  font-size: 0.82rem;
+}
+
+.form-error {
+  margin: 0;
+  color: #fca5a5;
+  font-size: 0.86rem;
+}
+
+.app-shell {
+  width: min(86rem, calc(100% - 2rem));
+  margin: 0 auto;
+  padding: 2rem 0;
+  display: grid;
+  grid-template-columns: 14rem minmax(0, 1fr);
+  gap: 1.5rem;
+}
+
+.app-sidebar {
+  position: sticky;
+  top: 5rem;
+  align-self: start;
+  display: grid;
+  gap: 0.5rem;
+}
+
+.sidebar-link {
+  padding: 0.62rem 0.75rem;
+  border: 1px solid transparent;
+  border-radius: var(--radius);
+  color: var(--muted-foreground);
+  text-decoration: none;
+}
+
+.sidebar-link:hover,
+.sidebar-link.is-active {
+  border-color: var(--border);
+  background: var(--muted);
+  color: var(--foreground);
+}
+
+.collection-create {
+  display: grid;
+  gap: 0.6rem;
+  margin-top: 1rem;
+}
+
+.app-main {
+  min-width: 0;
+  display: grid;
+  gap: 1rem;
+}
+
+.compact-upload .drop-zone {
+  min-height: 11rem;
+}
+
+.dashboard-options {
+  grid-template-columns: repeat(3, minmax(0, 1fr));
+}
+
+.collection-tabs,
+.inline-controls {
+  display: flex;
+  align-items: end;
+  flex-wrap: wrap;
+  gap: 0.65rem;
+}
+
+.inline-controls input,
+.inline-controls select {
+  min-width: 15rem;
+}
+
+.compact-input {
+  width: 10rem;
+}
+
 .drop-zone {
   min-height: 19rem;
   display: grid;