feat(server+ui): tag management API routes and browser UI

- 5 tag routes (list tree, create, assign, remove, search with tag=)
- Tags nav, tag tree view, tag filter badge
- Entry tag pills with remove, assign-tag form in context rail

crates/archivr-server/static/index.html

@@ -14,6 +14,7 @@
         <button class="nav-link is-active" data-view="archive">Archive</button>
         <button class="nav-link" data-view="runs">Runs</button>
         <button class="nav-link" data-view="admin">Admin</button>
+        <button class="nav-link" data-view="tags">Tags</button>
       </nav>
       <button class="capture-button">+ Capture</button>
     </header>
@@ -66,11 +67,20 @@
           <h1>Mounted Archives</h1>
           <div id="admin-archives" class="admin-list"></div>
         </section>
+
+        <section id="tags-view" class="view">
+          <div id="tag-tree" class="tag-tree"></div>
+        </section>
       </section>
 
       <aside class="context-rail">
         <div class="rail-title">Context</div>
         <div id="context-body" class="rail-body">Select an entry.</div>
+        <div id="entry-tags" class="entry-tags" hidden></div>
+        <div id="assign-tag-form" class="assign-tag-form" hidden>
+          <input id="assign-tag-input" class="assign-tag-input" type="text" placeholder="/science/cs" autocomplete="off">
+          <button id="assign-tag-btn" class="assign-tag-btn">Add tag</button>
+        </div>
       </aside>
     </main>