feat: planning pass counts all folders up front for account-wide progress

Before copying, EXAMINE every folder to sum the account's total message count
and emit a 'plan' event; progress events now carry account_total so the UI
shows a real overall bar, percent and ETA (not just per-folder).

- imapx.FolderMessageCount: read-only count of a folder
- orchestrator: plan pass + grandTotal, plan event, account_total in progress
- web: live progress keyed on account total; PLAN log line; overall bar/ETA

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MMHQTtnQtQqL8muAXHr9kd

internal/imapx/account.go

@@ -3,9 +3,21 @@ package imapx
 import (
 	"context"
 
+	"github.com/emersion/go-imap/v2"
 	"github.com/emersion/go-imap/v2/imapclient"
 )
 
+// FolderMessageCount opens a folder read-only (EXAMINE) and returns how many
+// messages it holds — used to plan an accurate overall progress total before
+// copying begins. It does not fetch any message bodies.
+func FolderMessageCount(c *imapclient.Client, folder string) (int64, error) {
+	sel, err := c.Select(folder, &imap.SelectOptions{ReadOnly: true}).Wait()
+	if err != nil {
+		return 0, err
+	}
+	return int64(sel.NumMessages), nil
+}
+
 // ListFolders returns the mailbox names visible on an already-connected, logged-in client.
 func ListFolders(c *imapclient.Client) ([]string, error) {
 	mboxes, err := c.List("", "*", nil).Collect()