Polish file viewer UX review punchlist (PAP-1962)

Addresses UX review feedback on PAP-1963:

- Fix denial-body leak for too_large/size codes (describeDenial no longer
  echoes fallback for known codes; the call site passes empty fallback so
  unknown codes show a generic message).
- Sticky line-number gutter with solid code-bg fill; highlight border
  moves from absolute-positioned sibling onto the sticky gutter so the
  amber marker stays pinned when users scroll long lines horizontally.
- Swap FileSearch copy-link icon for Link2.
- WorkspaceFileLink now sets a real deep-link href so modifier-click
  opens the file viewer in a new tab instead of scrolling to the top.
- Collapse duplicate title/description in the sheet header when the
  basename equals the full path (e.g. .env.production).
- Drop the duplicate title/helper block from OpenFilePrompt (sheet
  header already carries the title + description).
- Replace ArrowLeft with Cloud for the remote-workspace state.
- Retire arbitrary text-[12.5px]/[11px]/[0.78em] sizes in favor of text-xs.
- Unify 404 copy on "active workspace" everywhere.
- Raise chip-row contrast by using bg-muted instead of bg-background.

Also commits previously untracked visual-regression infra:
- Storybook stories for 12 file viewer states
- screenshot-file-viewer.mjs / screenshot-one.mjs
- describeDenial unit tests

Co-Authored-By: Paperclip <noreply@paperclip.ing>

scripts/screenshot-file-viewer.mjs

@@ -0,0 +1,73 @@
+#!/usr/bin/env node
+// Screenshots every FileViewerSheet Storybook state at desktop + mobile viewports.
+
+import fs from "node:fs/promises";
+import path from "node:path";
+import { chromium } from "@playwright/test";
+
+const BASE = "http://localhost:6006/iframe.html";
+const OUT_DIR = path.resolve(process.argv[2] || "/tmp/pap-1963");
+
+const SHOTS = [
+  { id: "components-file-viewer-sheet--text-content-with-highlighted-line", label: "content-highlighted-line" },
+  { id: "components-file-viewer-sheet--text-content-long-path-truncation", label: "content-long-path" },
+  { id: "components-file-viewer-sheet--open-file-prompt", label: "open-file-prompt" },
+  { id: "components-file-viewer-sheet--loading-spinner", label: "loading-spinner" },
+  { id: "components-file-viewer-sheet--error-not-found-with-fallback", label: "error-not-found" },
+  { id: "components-file-viewer-sheet--error-no-workspace", label: "error-no-workspace" },
+  { id: "components-file-viewer-sheet--error-outside-workspace", label: "error-outside-workspace" },
+  { id: "components-file-viewer-sheet--error-denied-sensitive", label: "error-denied" },
+  { id: "components-file-viewer-sheet--remote-workspace", label: "remote-workspace" },
+  { id: "components-file-viewer-sheet--workspace-archived", label: "workspace-archived" },
+  { id: "components-file-viewer-sheet--binary-unsupported", label: "binary-unsupported" },
+  { id: "components-file-viewer-sheet--too-large-to-preview", label: "too-large" },
+];
+
+const VIEWPORTS = [
+  { name: "desktop", width: 1440, height: 900 },
+  { name: "mobile", width: 390, height: 844 },
+];
+
+async function run() {
+  await fs.mkdir(OUT_DIR, { recursive: true });
+  const browser = await chromium.launch({ headless: true });
+  try {
+    for (const vp of VIEWPORTS) {
+      const context = await browser.newContext({
+        viewport: { width: vp.width, height: vp.height },
+        deviceScaleFactor: 2,
+      });
+      const page = await context.newPage();
+      for (const shot of SHOTS) {
+        const url = `${BASE}?id=${shot.id}&viewMode=story&args=`;
+        console.log(`[${vp.name}] → ${shot.label}`);
+        await page.goto(url, { waitUntil: "domcontentloaded" });
+        // Give the sheet animation + potential network stubs ~1.5s to settle.
+        await page.waitForTimeout(shot.label === "loading-spinner" ? 900 : 1600);
+        const out = path.join(OUT_DIR, `${vp.name}-${shot.label}.png`);
+        await page.screenshot({ path: out, fullPage: false });
+      }
+      // Extra capture: mobile view story (its own viewport story) as a cross-check.
+      if (vp.name === "mobile") {
+        await page.goto(
+          `${BASE}?id=components-file-viewer-sheet--mobile-view&viewMode=story&args=`,
+          { waitUntil: "domcontentloaded" },
+        );
+        await page.waitForTimeout(1600);
+        await page.screenshot({
+          path: path.join(OUT_DIR, `mobile-story-variant.png`),
+          fullPage: false,
+        });
+      }
+      await context.close();
+    }
+  } finally {
+    await browser.close();
+  }
+  console.log(`\nScreenshots written to ${OUT_DIR}`);
+}
+
+run().catch((error) => {
+  console.error(error);
+  process.exit(1);
+});

scripts/screenshot-one.mjs

@@ -0,0 +1,27 @@
+#!/usr/bin/env node
+// Screenshot a single story id at desktop 1440x900.
+import fs from "node:fs/promises";
+import path from "node:path";
+import { chromium } from "@playwright/test";
+
+const [, , storyId, out, widthArg, heightArg, waitArg] = process.argv;
+const width = Number(widthArg) || 1440;
+const height = Number(heightArg) || 900;
+const waitMs = Number(waitArg) || 2500;
+const url = `http://localhost:6006/iframe.html?id=${storyId}&viewMode=story`;
+
+await fs.mkdir(path.dirname(path.resolve(out)), { recursive: true });
+const browser = await chromium.launch({ headless: true });
+try {
+  const ctx = await browser.newContext({
+    viewport: { width, height },
+    deviceScaleFactor: 2,
+  });
+  const page = await ctx.newPage();
+  await page.goto(url, { waitUntil: "networkidle" });
+  await page.waitForTimeout(waitMs);
+  await page.screenshot({ path: out, fullPage: false });
+} finally {
+  await browser.close();
+}
+console.log(`Wrote ${out}`);