Replace emoji icons with a monochrome SVG icon set

Emoji (📚 🔖 🏷️ ✎ ✕ 🕓 ⏳ 🤖 ⚠️ 🔍) rendered in each platform's own
colorful style and looked inconsistent. Add app/templates/_icons.html
with small inline SVG macros (Lucide-style, MIT-licensed path data,
stroke=currentColor) and use them throughout instead, so icons pick
up the muted/accent text colors like the rest of the UI.

The needs-review bookmark toggle now uses an outline vs. filled icon
for its two states instead of swapping emoji.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

app/templates/base.html

@@ -128,6 +128,23 @@
     .htmx-indicator { opacity: 0; transition: opacity .2s; }
     .htmx-request .htmx-indicator { opacity: 1; }
     .htmx-request.htmx-indicator { opacity: 1; }
+
+    .icon { display: inline-block; vertical-align: -3px; flex-shrink: 0; }
+    .icon-spin { animation: icon-spin 0.8s linear infinite; }
+    @keyframes icon-spin { to { transform: rotate(360deg); } }
+    .icon-btn {
+      display: inline-flex; align-items: center; justify-content: center;
+      width: 30px; height: 30px; padding: 0;
+    }
+    .icon-warn { color: #eab308; }
+    .icon-bookmark-active { color: #eab308; }
+
+    .input-icon { position: relative; }
+    .input-icon > .icon {
+      position: absolute; left: 12px; top: 50%; transform: translateY(-50%);
+      color: var(--muted); pointer-events: none;
+    }
+    .input-icon > input { padding-left: 36px; }
     footer.app-footer {
       text-align: center; padding: 16px 24px; margin-top: 24px;
       color: var(--muted); font-size: .8rem; border-top: 1px solid var(--border);