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/themes/base.css

@@ -9,7 +9,7 @@
   --series-6: #008300;
   --series-7: #9085e9;
   --series-8: #e66767;
-  --seq-0: var(--bg1);
+  --seq-0: color-mix(in srgb, var(--fg1) 9%, var(--bg1));
   --seq-1: #104281;
   --seq-2: #184f95;
   --seq-3: #256abf;
@@ -26,7 +26,7 @@
   --series-6: #008300;
   --series-7: #6250d6;
   --series-8: #e34948;
-  --seq-0: var(--bg1);
+  --seq-0: color-mix(in srgb, var(--fg1) 9%, var(--bg1));
   --seq-1: #cde2fb;
   --seq-2: #9ec5f4;
   --seq-3: #6da7ec;