Add hooks, @file mentions, session search/fork/export, follow-scroll

- Hooks ([[hooks]]): pre_tool, post_tool, user_prompt, stop; sandboxed, exit 2 blocks
- @file mentions with autocomplete; mentioned files/folders are attached to the prompt
- Session full-text search in the sidebar, fork into a copy, export as Markdown/JSON
- Chat follows the newest message until the user scrolls up; jump-to-latest button
- SVG activity spark and thinking icon instead of Unicode/emoji glyphs
- File search ranks name matches first and returns relative paths

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

web/src/styles.css

@@ -1362,6 +1362,8 @@ a.button:hover {
 @media (prefers-reduced-motion: reduce) {
   .dots i,
   .spinner,
+  .star line,
+  .star-spin,
   .busy-ring,
   .tool.running,
   .streaming .markdown > :last-child::after,
@@ -1964,11 +1966,43 @@ table.grid.sheet th {
 }
 
 .star {
-  color: var(--accent2);
-  font-size: 15px;
-  width: 1em;
-  display: inline-block;
-  text-align: center;
+  width: 16px;
+  height: 16px;
+  flex-shrink: 0;
+  vertical-align: -3px;
+  overflow: visible;
+}
+.star line {
+  stroke: var(--accent2);
+  stroke-width: 2.6;
+  stroke-linecap: round;
+  opacity: 0.25;
+  animation: star-ray 0.9s linear infinite;
+}
+.star-spin {
+  transform-origin: 0 0;
+  animation: spin 6s linear infinite;
+}
+@keyframes star-ray {
+  0% {
+    opacity: 1;
+  }
+  100% {
+    opacity: 0.2;
+  }
+}
+.think-icon {
+  width: 15px;
+  height: 15px;
+  vertical-align: -2px;
+  fill: none;
+  stroke: var(--fg3);
+  stroke-width: 1.6;
+  stroke-linejoin: round;
+}
+.think-icon circle {
+  fill: var(--fg3);
+  stroke: none;
 }
 .activity-text {
   color: var(--accent2);
@@ -2204,3 +2238,82 @@ button.danger-solid {
   align-items: center;
   justify-content: flex-start;
 }
+
+/* --- jump to latest message ---------------------------------------------------- */
+.jump-latest {
+  position: sticky;
+  bottom: 14px;
+  height: 0;
+  display: flex;
+  justify-content: center;
+  pointer-events: none;
+  z-index: 4;
+}
+.jump-latest button {
+  pointer-events: auto;
+  transform: translateY(-100%);
+  width: 40px;
+  height: 40px;
+  border-radius: 50%;
+  font-size: 20px;
+  line-height: 1;
+  padding: 0;
+  box-shadow: var(--shadow);
+}
+
+/* --- session search -------------------------------------------------------------- */
+.session-search {
+  padding: 0 12px 8px;
+}
+.session-search input {
+  width: 100%;
+}
+.session-item .snippet {
+  white-space: normal;
+  display: -webkit-box;
+  -webkit-line-clamp: 2;
+  -webkit-box-orient: vertical;
+  overflow: hidden;
+  color: var(--fg2);
+}
+
+/* --- small popup menu (export) ---------------------------------------------- */
+.menu-pop {
+  position: relative;
+}
+.menu-pop > summary {
+  list-style: none;
+  cursor: pointer;
+  padding: 4px 8px;
+  border-radius: 6px;
+}
+.menu-pop > summary::-webkit-details-marker {
+  display: none;
+}
+.menu-pop > summary:hover {
+  background: var(--bg2);
+}
+.menu-pop-body {
+  position: absolute;
+  right: 0;
+  top: calc(100% + 4px);
+  z-index: 20;
+  background: var(--bg1);
+  border: 1px solid var(--bg3);
+  border-radius: var(--radius);
+  box-shadow: var(--shadow);
+  display: flex;
+  flex-direction: column;
+  min-width: 160px;
+  padding: 4px;
+}
+.menu-pop-body a {
+  padding: 6px 10px;
+  border-radius: 6px;
+  color: var(--fg1);
+  text-decoration: none;
+  white-space: nowrap;
+}
+.menu-pop-body a:hover {
+  background: var(--bg2);
+}