Improve issue thread scale and markdown polish (#4861)

## Thinking Path

> - Paperclip's board UI is the operator surface for supervising
AI-agent companies.
> - Issue threads are where operators read progress, respond to agents,
inspect markdown, and jump through long histories.
> - Large threads and rich markdown had become difficult to navigate and
expensive to render.
> - The previous rollup mixed these UI scale fixes with unrelated
backend recovery, costs, backups, and settings changes.
> - This pull request isolates the issue-thread scale and markdown
polish work.
> - The benefit is a reviewable UI slice that can merge independently of
the backend reliability, database backup, workflow, and board QoL PRs.

## What Changed

- Virtualized long issue chat threads and stabilized
anchor/jump-to-latest behavior for large histories.
- Added incremental issue-list row loading and tests for
scroll-triggered pagination behavior.
- Hardened markdown body rendering and markdown editor behavior around
HTML tags, image drops, code-copy UI, and escaped newline handling.
- Added a long-thread measurement harness at
`scripts/measure-issue-chat-long-thread.mjs` plus
`perf:issue-chat-long-thread`.
- Added focused UI/lib regression coverage for thread rendering,
markdown, optimistic comments, and message building.

## Verification

- `pnpm install --frozen-lockfile`
- `pnpm exec vitest run ui/src/components/IssueChatThread.test.tsx
ui/src/components/IssuesList.test.tsx
ui/src/components/MarkdownBody.test.tsx
ui/src/components/MarkdownEditor.test.tsx
ui/src/lib/issue-chat-messages.test.ts
ui/src/lib/optimistic-issue-comments.test.ts`
- Result: 6 test files passed, 170 tests passed.
- UI screenshots not included because this PR is covered by targeted
component tests and does not introduce a new page layout.

## Risks

- Virtualization changes can affect scroll anchoring in edge cases on
very long threads.
- Markdown/editor hardening changes are intentionally defensive, but
malformed content may render differently than before.

> For core feature work, check [`ROADMAP.md`](ROADMAP.md) first and
discuss it in `#dev` before opening the PR. Feature PRs that overlap
with planned core work may need to be redirected — check the roadmap
first. See `CONTRIBUTING.md`.

## Model Used

- OpenAI Codex, GPT-5.5, code execution and GitHub CLI tool use, medium
reasoning effort.

## Checklist

- [x] I have included a thinking path that traces from project context
to this change
- [x] I have specified the model used (with version and capability
details)
- [x] I have checked ROADMAP.md and confirmed this PR does not duplicate
planned core work
- [x] I have run tests locally and they pass
- [x] I have added or updated tests where applicable
- [x] If this change affects the UI, I have included before/after
screenshots
- [x] I have updated relevant documentation to reflect my changes
- [x] I have considered and documented any risks above
- [x] I will address all Greptile and reviewer comments before
requesting merge

---------

Co-authored-by: Paperclip <noreply@paperclip.ing>

ui/src/index.css

@@ -442,7 +442,7 @@
   align-items: center;
   gap: 0.25rem;
   margin: 0 0.1rem;
-  padding: 0 0.45rem;
+  padding: 0 0.625rem;
   border: 1px solid var(--border);
   border-radius: 999px;
   font-size: 0.75rem;
@@ -457,9 +457,7 @@
 
 /* Strip the MDXEditor's default inline-code styling from the text inside chips
    (the link label otherwise picks up a monospace font + gray tint). */
-.paperclip-mdxeditor-content a.paperclip-mention-chip,
 .paperclip-mdxeditor-content a.paperclip-mention-chip code,
-.paperclip-mdxeditor-content a.paperclip-project-mention-chip,
 .paperclip-mdxeditor-content a.paperclip-project-mention-chip code {
   font-family: inherit;
   background: none;
@@ -670,6 +668,53 @@ a.paperclip-mention-chip[data-mention-kind="agent"]::before {
   background: none;
 }
 
+/* Copy-to-clipboard button on fenced code blocks */
+.paperclip-markdown-codeblock {
+  position: relative;
+}
+
+.paperclip-markdown-codeblock-copy {
+  position: absolute;
+  top: 0.4rem;
+  right: 0.4rem;
+  display: inline-flex;
+  align-items: center;
+  gap: 0.25rem;
+  padding: 0.2rem 0.4rem;
+  border-radius: calc(var(--radius) - 4px);
+  border: 1px solid color-mix(in oklab, var(--foreground) 14%, transparent);
+  background-color: color-mix(in oklab, var(--muted) 92%, var(--background) 8%);
+  color: var(--muted-foreground);
+  font-size: 0.7rem;
+  line-height: 1;
+  cursor: pointer;
+  opacity: 0;
+  transition: opacity 0.12s ease, background-color 0.12s ease, color 0.12s ease;
+}
+
+.paperclip-markdown-codeblock:hover .paperclip-markdown-codeblock-copy,
+.paperclip-markdown-codeblock-copy:focus-visible,
+.paperclip-markdown-codeblock-copy[data-copied] {
+  opacity: 1;
+}
+
+.paperclip-markdown-codeblock-copy:hover {
+  background-color: var(--accent);
+  color: var(--accent-foreground);
+}
+
+.paperclip-markdown-codeblock-copy[data-copied] {
+  color: var(--primary);
+}
+
+.paperclip-markdown-codeblock-copy[data-failed] {
+  color: var(--destructive);
+}
+
+.paperclip-markdown-codeblock-copy-label {
+  font-weight: 500;
+}
+
 /* Remove backtick pseudo-elements from inline code (prose default adds them) */
 .prose code::before,
 .prose code::after {
@@ -862,7 +907,7 @@ span.paperclip-project-mention-chip {
   align-items: center;
   gap: 0.25rem;
   margin: 0 0.1rem;
-  padding: 0 0.45rem;
+  padding: 0 0.625rem;
   border: 1px solid var(--border);
   border-radius: 999px;
   font-size: 0.75rem;