feat(site): display file attachments in chat UI (#24281)

Renders the durable file attachments introduced in #24280 in the chat
interface. Without this, attachments were stored and served correctly
but the UI showed raw file parts with no previews or download UX.

Every attachment gets a download affordance, split into three rendering
tiers:

- **Images** — thumbnail with a hover/focus overlay containing a
download link. `onFocusCapture`/`onBlurCapture` with
`contains(relatedTarget)` keeps the overlay open while tabbing between
the image and its download link.
- **Text-like files** (`text/*`, `application/json`) — expandable
preview button with loading + error-with-retry states and the same
download overlay. Preview fetches throw a typed
`FetchTextAttachmentError` with a `.status` field instead of a
stringly-typed error.
- **Everything else** — compact `FileCard` with extension badge,
filename, and download link.

User-side and assistant-side rendering now share `AttachmentBlocks.tsx`
(`AttachmentPreviewFrame`, `TextAttachmentButton`,
`ImageAttachmentButton`, `FileCard`, plus
`getAttachmentHref`/`getAttachmentName`) instead of two near-duplicate
implementations. The text-attachment overlay anchors to the preview
surface so the download button stays pinned even when a loading/error
status line widens the row below.

`ComputerRenderer` detects when a screenshot was stored as a durable
attachment (`attachment_file_id`) and suppresses the stale base64
rendering — the screenshot appears as a proper file part instead.
`ToolLabel` shows the attached filename for `attach_file` tool calls.

Storybook coverage in `ConversationTimeline.stories.tsx` was expanded to
cover every tier (single/multiple images, inline + file-id text, JSON,
download-only files, fetch-failure retry, mixed attachments + file
references) with play-function assertions.

<img width="811" height="150" alt="image"
src="https://github.com/user-attachments/assets/27c71081-3502-4e80-92a7-d8adf1ff9323"
/>



## Cleanup

Per Mathias' post-merge suggestion on #24280, this PR also relocates
`coderd/chatfiles` → `coderd/x/chatfiles` so the durable-attachment
helpers live beside the rest of the `chatd` experimental surface.

Closes CODAGT-91

coderd/x/chatd/chatloop/chatloop.go

@@ -19,11 +19,13 @@ import (
 	"charm.land/fantasy/schema"
 	"golang.org/x/xerrors"
 
+	"cdr.dev/slog/v3"
 	"github.com/coder/coder/v2/coderd/database/dbtime"
 	"github.com/coder/coder/v2/coderd/x/chatd/chatdebug"
 	"github.com/coder/coder/v2/coderd/x/chatd/chaterror"
 	"github.com/coder/coder/v2/coderd/x/chatd/chatprompt"
 	"github.com/coder/coder/v2/coderd/x/chatd/chatretry"
+	"github.com/coder/coder/v2/coderd/x/chatd/chattool"
 	"github.com/coder/coder/v2/codersdk"
 	"github.com/coder/quartz"
 )
@@ -146,6 +148,7 @@ type RunOptions struct {
 		role codersdk.ChatMessageRole,
 		part codersdk.ChatMessagePart,
 	)
+	Logger           slog.Logger
 	Compaction       *CompactionOptions
 	ReloadMessages   func(context.Context) ([]fantasy.Message, error)
 	DisableChainMode func()
@@ -492,7 +495,8 @@ func Run(ctx context.Context, opts RunOptions) error {
 				// Execute only built-in tools.
 				toolResults = executeTools(ctx, opts.Tools, opts.ActiveTools, opts.ProviderTools, builtinCalls, opts.Metrics, provider, modelName, opts.BuiltinToolNames, func(tr fantasy.ToolResultContent, completedAt time.Time) {
 					recordToolResultTimestamp(&result, tr.ToolCallID, completedAt)
-					ssePart := chatprompt.PartFromContent(tr)
+					publishToolAttachments(ctx, opts.Logger, tr, completedAt, publishMessagePart)
+					ssePart := chatprompt.PartFromContentWithLogger(ctx, opts.Logger, tr)
 					ssePart.CreatedAt = &completedAt
 					publishMessagePart(codersdk.ChatMessageRoleTool, ssePart)
 				})
@@ -1545,6 +1549,33 @@ func recordToolResultTimestamp(result *stepResult, toolCallID string, ts time.Ti
 	result.toolResultCreatedAt[toolCallID] = ts
 }
 
+func publishToolAttachments(
+	ctx context.Context,
+	logger slog.Logger,
+	tr fantasy.ToolResultContent,
+	createdAt time.Time,
+	publishMessagePart func(codersdk.ChatMessageRole, codersdk.ChatMessagePart),
+) {
+	attachments, err := chattool.AttachmentsFromMetadata(tr.ClientMetadata)
+	if err != nil {
+		logger.Warn(ctx, "skipping malformed tool attachment metadata",
+			slog.F("tool_name", tr.ToolName),
+			slog.F("tool_call_id", tr.ToolCallID),
+			slog.Error(err),
+		)
+		return
+	}
+	for _, attachment := range attachments {
+		filePart := codersdk.ChatMessageFile(
+			attachment.FileID,
+			attachment.MediaType,
+			attachment.Name,
+		)
+		filePart.CreatedAt = &createdAt
+		publishMessagePart(codersdk.ChatMessageRoleAssistant, filePart)
+	}
+}
+
 func extractContextLimit(metadata fantasy.ProviderMetadata) sql.NullInt64 {
 	if len(metadata) == 0 {
 		return sql.NullInt64{}