fix(web): restore markdown file-link CSS lost during merge

Today's merge from main silently dropped several CSS additions that
landed on main while our branch was in flight:

- `.chat-markdown-file-link` / `:hover` / `:focus-visible` /
  `-icon` / `-label` (from #1956 — the file-link UX redesign)
- `.markdown-file-link-tooltip-scroll` + its `::-webkit-scrollbar*`
  variants (same PR)
- `--chat-markdown-codeblock-copy-button-space` var, the padding-right
  adjustment that uses it, and the `cursor: pointer` on the copy
  button (from #1985)
- `@custom-variant wco` (from #1969)

`ChatMarkdown.tsx` still references all four file-link class names, so
their absence silently un-styled every inline file link the assistant
renders. Fix by rebasing our index.css additions on top of origin/main
so the only delta versus main is the additive Chat-font-size tier
block (`text-chat-*` utilities + `[data-timeline-root]` scaling).

Author: akarabach

apps/web/src/index.css

@@ -1,6 +1,8 @@
 @import "tailwindcss";
 
 @custom-variant dark (&:is(.dark, .dark *));
+/* Window Controls Overlay: active when Electron exposes native titlebar control geometry. */
+@custom-variant wco (&:is(.wco, .wco *));
 
 @theme inline {
   --animate-skeleton: skeleton 2s -1s infinite linear;
@@ -360,6 +362,47 @@ label:has(> select#reasoning-effort) select {
   font-size: 0.75rem;
 }
 
+.chat-markdown-file-link {
+  display: inline-flex;
+  align-items: center;
+  gap: 0.28rem;
+  border: 1px solid color-mix(in srgb, var(--border) 92%, transparent);
+  border-radius: 0.375rem;
+  background: color-mix(in srgb, var(--muted) 88%, var(--background));
+  padding: 0.08rem 0.34rem;
+  color: var(--foreground);
+  font-family: var(--font-mono, ui-monospace, monospace);
+  font-size: 0.75rem;
+  line-height: 1.15;
+  vertical-align: text-bottom;
+  transition:
+    background-color 120ms ease,
+    border-color 120ms ease,
+    color 120ms ease,
+    opacity 120ms ease;
+}
+
+.chat-markdown-file-link:hover {
+  opacity: 1;
+  color: var(--foreground);
+  border-color: color-mix(in srgb, var(--border) 65%, var(--foreground));
+  background: color-mix(in srgb, var(--muted) 72%, var(--background));
+}
+
+.chat-markdown-file-link:focus-visible {
+  outline: none;
+  box-shadow: 0 0 0 1px color-mix(in srgb, var(--ring) 70%, transparent);
+}
+
+.chat-markdown-file-link-icon {
+  opacity: 0.72;
+}
+
+.chat-markdown-file-link-label {
+  color: color-mix(in srgb, var(--foreground) 88%, transparent);
+  line-height: 1.1;
+}
+
 .chat-markdown pre {
   max-width: 100%;
   overflow-x: auto;
@@ -376,13 +419,33 @@ label:has(> select#reasoning-effort) select {
   font-size: 0.75rem;
 }
 
+.markdown-file-link-tooltip-scroll {
+  scrollbar-width: thin;
+  scrollbar-color: color-mix(in srgb, var(--border) 78%, transparent) transparent;
+}
+
+.markdown-file-link-tooltip-scroll::-webkit-scrollbar {
+  height: 6px;
+}
+
+.markdown-file-link-tooltip-scroll::-webkit-scrollbar-track {
+  background: transparent;
+}
+
+.markdown-file-link-tooltip-scroll::-webkit-scrollbar-thumb {
+  border-radius: 999px;
+  background: color-mix(in srgb, var(--border) 78%, transparent);
+}
+
 .chat-markdown .chat-markdown-codeblock {
+  --chat-markdown-codeblock-copy-button-space: 1.5rem;
   position: relative;
   margin: 0.65rem 0;
 }
 
 .chat-markdown .chat-markdown-codeblock pre {
   margin: 0;
+  padding-right: calc(0.9rem + var(--chat-markdown-codeblock-copy-button-space));
 }
 
 .chat-markdown .chat-markdown-copy-button {
@@ -399,6 +462,7 @@ label:has(> select#reasoning-effort) select {
   border: 1px solid var(--border);
   background: color-mix(in srgb, var(--background) 82%, transparent);
   color: var(--muted-foreground);
+  cursor: pointer;
   opacity: 0;
   pointer-events: none;
   transition: