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>

ui/src/components/WorkspaceFileLink.tsx

@@ -1,9 +1,13 @@
 import type { MouseEvent, ReactNode } from "react";
 import { FileCode2 } from "lucide-react";
+import { useLocation } from "@/lib/router";
 import { cn } from "@/lib/utils";
 import type { ParsedWorkspaceFileRef } from "@/lib/workspace-file-parser";
 import { formatWorkspaceFileRefDisplay } from "@/lib/workspace-file-parser";
-import { useFileViewer } from "@/context/FileViewerContext";
+import {
+  useFileViewer,
+  writeFileViewerStateToSearch,
+} from "@/context/FileViewerContext";
 
 export interface WorkspaceFileLinkProps {
   workspaceFileRef: ParsedWorkspaceFileRef;
@@ -25,6 +29,7 @@ export function WorkspaceFileLink({
   title,
 }: WorkspaceFileLinkProps) {
   const viewer = useFileViewer();
+  const location = useLocation();
   const display = typeof label !== "undefined" ? label : formatWorkspaceFileRefDisplay(workspaceFileRef);
   const canOpen = !!(onOpen || viewer);
   const lineSuffix = workspaceFileRef.line
@@ -37,10 +42,21 @@ export function WorkspaceFileLink({
     ? `Open ${workspaceFileRef.path}${lineSuffix} in the file viewer`
     : `Workspace file ${workspaceFileRef.path}${lineSuffix}`);
 
+  const deepLinkSearch = writeFileViewerStateToSearch(location.search, {
+    path: workspaceFileRef.path,
+    line: workspaceFileRef.line ?? null,
+    column: workspaceFileRef.column ?? null,
+    workspace: "auto",
+  });
+  const href = canOpen
+    ? `${location.pathname}${deepLinkSearch}${location.hash}`
+    : "#";
+
   const handleClick = (event: MouseEvent<HTMLAnchorElement>) => {
     if (event.metaKey || event.ctrlKey || event.shiftKey || event.altKey) {
       return;
     }
+    if (event.button !== 0) return;
     if (!canOpen) return;
     event.preventDefault();
     if (onOpen) onOpen(workspaceFileRef);
@@ -49,14 +65,14 @@ export function WorkspaceFileLink({
 
   return (
     <a
-      href="#"
+      href={href}
       role={canOpen ? "button" : undefined}
       data-workspace-file-link="true"
       data-workspace-file-path={workspaceFileRef.path}
       aria-label={ariaLabel}
       title={tooltip}
       className={cn(
-        "paperclip-workspace-file-link inline-flex items-center gap-1 rounded-sm border border-border bg-muted/60 px-1.5 py-0.5 font-mono text-[0.78em] leading-tight text-foreground/90 align-baseline no-underline hover:bg-muted focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-ring focus-visible:ring-offset-1",
+        "paperclip-workspace-file-link inline-flex items-center gap-1 rounded-sm border border-border bg-muted/60 px-1.5 py-0.5 font-mono text-xs leading-tight text-foreground/90 align-baseline no-underline hover:bg-muted focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-ring focus-visible:ring-offset-1",
         className,
       )}
       onClick={handleClick}