Add file badges, menu icons, LSP diagnostics, robust compaction

- Files in chat: preview with type/size badges and file name; attachment chips
- Menu and settings tabs with icons
- Heatmap: distinct colour for empty hours, hourly labels
- Context chip shows used/total tokens (e.g. 221k/240k) plus percent
- LSP client: diagnostics appended to edit/write results and a diagnostics tool
  (typescript-language-server, pyright, rust-analyzer, gopls, clangd; sandboxed)
- Compaction summarizes a plain transcript and retries once on empty output
- files/stat endpoint

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

web/src/components/Composer.tsx

@@ -1,3 +1,4 @@
+import { formatSize } from "./FileFigure";
 import { useEffect, useMemo, useRef, useState } from "react";
 import { useAgents, useFetch } from "../hooks";
 import { useT, type Key } from "../i18n";
@@ -17,11 +18,6 @@ const BUILTIN: { name: string; key: Key }[] = [
 
 const MAX_SIDE = 2000;
 
-function formatSize(n: number) {
-  if (n < 1024) return `${n} B`;
-  if (n < 1024 * 1024) return `${(n / 1024).toFixed(1)} KB`;
-  return `${(n / 1024 / 1024).toFixed(1)} MB`;
-}
 
 /** Read an image and downscale it: models don't use more detail, and uploads stay small. */
 async function readImage(file: File): Promise<ImageData> {