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/lib/issue-chat-messages.test.ts

@@ -605,6 +605,37 @@ describe("buildIssueChatMessages", () => {
     });
   });
 
+  it("labels pause-caused cancelled runs as paused by board", () => {
+    const messages = buildIssueChatMessages({
+      comments: [],
+      timelineEvents: [],
+      linkedRuns: [
+        {
+          runId: "run-paused",
+          status: "cancelled",
+          agentId: "agent-1",
+          agentName: "CodexCoder",
+          createdAt: new Date("2026-04-06T12:01:00.000Z"),
+          startedAt: new Date("2026-04-06T12:01:00.000Z"),
+          finishedAt: new Date("2026-04-06T12:02:00.000Z"),
+          resultJson: { stopReason: "paused" },
+        },
+      ],
+      liveRuns: [],
+      transcriptsByRunId: new Map([
+        ["run-paused", [{ kind: "assistant", ts: "2026-04-06T12:01:05.000Z", text: "Working on it." }]],
+      ]),
+      hasOutputForRun: (runId) => runId === "run-paused",
+      currentUserId: "user-1",
+    });
+
+    expect(messages).toHaveLength(1);
+    expect(messages[0]?.metadata.custom).toMatchObject({
+      chainOfThoughtLabel: "Paused by board after 1 minute",
+      runStatus: "cancelled",
+    });
+  });
+
   it("can keep succeeded runs without transcript output for embedded run feeds", () => {
     const messages = buildIssueChatMessages({
       comments: [],