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>

server/src/tools/mod.rs

@@ -159,6 +159,7 @@ pub fn builtin_tools() -> Vec<Arc<dyn Tool>> {
         Arc::new(crate::shells::KillShell),
         Arc::new(web::WebFetch),
         Arc::new(web::WebSearch),
+        Arc::new(crate::lsp::Diagnostics),
         Arc::new(agent::TodoWrite),
         Arc::new(agent::AskUser),
         Arc::new(agent::Task),