Attach a file that is already in Files

Attaching meant uploading, even when the file was sitting in the account
already -- picking it off disk again to send the server a copy of what it
was holding.

The composer can now attach from Files. A blob the account can already
see needs no upload at all: an attachment carrying a `blobId` is what a
forward produces, so the send path has always known what to do with one.
Attaching a large file the server is already storing now costs nothing
and takes no time.

A file in an account somebody *shared* is different, because blobs belong
to the account they were uploaded to and a draft in yours cannot
reference one in theirs. Those are fetched and uploaded to your account,
and the picker says so before you attach rather than leaving someone
wondering why one file was instant and another was not.

The picker borrows the Files store, so it browses what Files browses,
shared accounts included, and puts the file manager back where it was on
the way out -- a detour through somebody's shared folder to find an
attachment should not leave Files somewhere else afterwards.

Verified against the mock, and worth recording how, because the first
attempt measured nothing: `client.upload` uses XMLHttpRequest, since it
reports progress, so a counter wrapped around `fetch` sees no uploads
whether or not any happen and agrees with you either way. Counted at
XHR instead: attaching one's own file issues no upload, and attaching a
shared one issues exactly one, to the reader's own account.

web/src/views/compose/Composer.tsx

@@ -1,5 +1,5 @@
 import { useCallback, useEffect, useMemo, useRef, useState } from "react";
-import { AlertTriangle, ChevronDown, FileText, Maximize2, Minimize2, Minus, MoreVertical, Paperclip, Send, Trash2, X, Type, Clock, CheckCheck, ChevronsDown } from "lucide-react";
+import { AlertTriangle, ChevronDown, FileText, FolderOpen, Maximize2, Minimize2, Minus, MoreVertical, Paperclip, Send, Trash2, X, Type, Clock, CheckCheck, ChevronsDown } from "lucide-react";
 import { useCompose, type Draft } from "@/store/compose";
 import { useMail } from "@/store/mail";
 import { useSettings } from "@/store/settings";
@@ -12,6 +12,8 @@ import { formatSize, formatRelative } from "@/lib/format";
 import { htmlToText, textToHtml } from "@/lib/text";
 import { isValidEmail } from "@/lib/address";
 import { attachmentIcon } from "../mail/MessageView";
+import { FilePicker } from "./FilePicker";
+import { useFiles } from "@/store/files";
 import { keyboard } from "@/lib/keyboard";
 import { useIsMobile } from "@/ui/misc";
 import { toast } from "@/ui/toast";
@@ -25,6 +27,9 @@ export function Composer({ draft }: { draft: Draft }) {
   const send = useCompose((s) => s.send);
   const saveDraft = useCompose((s) => s.saveDraft);
   const addFiles = useCompose((s) => s.addFiles);
+  const addFromFiles = useCompose((s) => s.addFromFiles);
+  const filesAvailable = useFiles((s) => s.available);
+  const [pickerOpen, setPickerOpen] = useState(false);
   const removeAttachment = useCompose((s) => s.removeAttachment);
   const setIdentity = useCompose((s) => s.setIdentity);
   const insertTemplate = useCompose((s) => s.insertTemplate);
@@ -239,11 +244,13 @@ export function Composer({ draft }: { draft: Draft }) {
             <MenuItem icon={<Clock size={16} />} label={`Undo window: ${settings.undoSendSeconds}s`} onClick={() => updateSettings({ undoSendSeconds: settings.undoSendSeconds >= 30 ? 0 : settings.undoSendSeconds + 5 })} />
             {canSchedule && <ScheduleMenuItems maxMs={scheduleMax} onPick={scheduleFor} onCustom={() => { sendMenu.close(); setScheduleOpen(true); }} />}
           </Popover>
+          {pickerOpen && <FilePicker onPick={(picked) => void addFromFiles(key, picked)} onClose={() => setPickerOpen(false)} />}
           {canSchedule && scheduleOpen && (
             <ScheduleDialog open maxMs={scheduleMax} initial={d.sendAt} onClose={() => setScheduleOpen(false)} onPick={scheduleFor} />
           )}
           <span className="more-actions">
             <button className="icon-btn" title="Attach files" onClick={() => fileRef.current?.click()}><Paperclip size={18} /></button>
+            {filesAvailable && <button className="icon-btn" title="Attach from Files" onClick={() => setPickerOpen(true)}><FolderOpen size={18} /></button>}
             <input ref={fileRef} type="file" multiple hidden onChange={(e) => { const files = Array.from(e.target.files ?? []); if (files.length) addFiles(key, files); e.target.value = ""; }} />
             {d.format === "html" && <button className={`icon-btn ${showToolbar ? "active" : ""}`} title="Formatting options" onClick={() => setShowToolbar((v) => !v)}><Type size={18} /></button>}
             {settings.templates.length > 0 && <button className="icon-btn" title="Insert template" onClick={templateMenu.open}><FileText size={18} /></button>}