feat: Lucide SVG icons throughout main UI

Replace all emoji/unicode icons with Lucide SVG icons:
- Mode select dropdown: message-circle / pencil / lock / bot
- Send button: arrow-up (chat/OTR), pencil (note), zap (agent)
- Stop button: square icon
- Header nav already had Lucide SVGs; render_icons() now called at init

Add icon_html() + render_icons() helpers; update update_mode_ui() and
open_mode_dropdown() to use innerHTML + lucide.createIcons(). CSS: .btn-icon
alignment, inline-flex on .hdr-btn / .hdr-dd-item / #send / #stop.

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

cortex/static/style.css

@@ -183,7 +183,13 @@
 
         .persona-dropdown .pd-add:hover { color: var(--text); }
 
+        /* Lucide SVG icon alignment */
+        .btn-icon { display: inline-block; vertical-align: middle; flex-shrink: 0; pointer-events: none; }
+
         .hdr-btn {
+            display: inline-flex;
+            align-items: center;
+            gap: 5px;
             background: var(--bg);
             border: 1px solid var(--border);
             border-radius: 6px;
@@ -224,7 +230,9 @@
         .hdr-dropdown.open { display: block; }
 
         .hdr-dd-item {
-            display: block;
+            display: flex;
+            align-items: center;
+            gap: 8px;
             width: 100%;
             text-align: left;
             padding: 0.55rem 0.85rem;
@@ -538,7 +546,7 @@
         #mode-select-btn.mode-otr               { border-color: rgba(120,80,160,0.6); color: #a87fd4; }
         #mode-select-btn.mode-agent             { border-color: rgba(80,140,200,0.6); color: #7cb9e8; }
 
-        #mode-icon  { font-size: 1rem; line-height: 1; }
+        #mode-icon  { display: flex; align-items: center; }
         .mode-arrow { font-size: 0.55rem; color: var(--muted); margin-left: 2px; opacity: 0.5; }
 
         /* Dropdown — opens upward; MRU at bottom = closest to button */
@@ -573,7 +581,7 @@
         }
         .mode-option:hover           { background: var(--border); color: var(--text); }
         .mode-option.current         { color: var(--text); font-weight: 500; }
-        .mode-option .opt-icon       { font-size: 1rem; line-height: 1; }
+        .mode-option .opt-icon       { display: flex; align-items: center; }
         .mode-option .opt-check      { margin-left: auto; font-size: 0.7rem; opacity: 0.7; }
 
         /* Note visibility sub-button — shown below mode-select when note is active */
@@ -630,6 +638,10 @@
 
         /* Send button */
         #send {
+            display: flex;
+            align-items: center;
+            justify-content: center;
+            gap: 6px;
             background: var(--user-bg);
             border: 1px solid var(--user-border);
             color: var(--text);
@@ -649,11 +661,14 @@
         /* Stop button */
         #stop {
             display: none;
+            align-items: center;
+            justify-content: center;
+            gap: 6px;
             background: var(--error-bg);
             border: 1px solid var(--error-border);
             color: var(--error-text);
             border-radius: 8px;
-            padding: 10px 0;
+            padding: 10px 14px;
             cursor: pointer;
             font-size: 0.9rem;
             text-align: center;