refactor: remove container selection, extract helpers, add logging and tests

- Remove container picker UI, keyboard, settings, and ContainerOptions
  entirely — yt-dlp handles container format automatically
- Remove Container field from DownloadJob struct and related references
- Extract buildFormatList helper to deduplicate format list building
  between handleURLInput and handleBackStep
- Break runDownload into trackDownloadProgress + handleDownloadCompletion
- Add HTTP timeout (30s) to downloadFile helper
- Log applyQuota and AddHistory errors instead of discarding
- Log nil stepState warnings in all handler entry points
- Include job.StderrLog in download failure messages
- Add tests for buildFormatList, findFormatByID, determineMediaType,
  and buildFormatString
- Update README with search, DRM fallback, and quota feature docs
- Remove unused strings import from keyboard.go

internal/handler/keyboard.go

@@ -2,7 +2,6 @@ package handler
 
 import (
 	"fmt"
-	"strings"
 
 	"github.com/go-telegram/bot/models"
 
@@ -66,25 +65,6 @@ func languageKeyboard(languages []string) models.InlineKeyboardMarkup {
 	return formatKeyboard("lang_", languages, languages)
 }
 
-// containerKeyboard builds keyboard for container format selection.
-func containerKeyboard(containers []string) models.InlineKeyboardMarkup {
-	rows := [][]models.InlineKeyboardButton{}
-	row := []models.InlineKeyboardButton{}
-	for i, c := range containers {
-		data := "container_" + c
-		row = append(row, models.InlineKeyboardButton{Text: strings.ToUpper(c), CallbackData: data})
-		if len(row) == 3 || i == len(containers)-1 {
-			rows = append(rows, row)
-			row = nil
-		}
-	}
-	rows = append(rows, []models.InlineKeyboardButton{
-		{Text: "Back", CallbackData: "back_step"},
-		{Text: "Cancel", CallbackData: "cancel_dl"},
-	})
-	return models.InlineKeyboardMarkup{InlineKeyboard: rows}
-}
-
 // progressKeyboard builds the progress display with a dummy button and cancel.
 func progressKeyboard(pct float64, speed, eta string) models.InlineKeyboardMarkup {
 	progressText := fmt.Sprintf("%.1f%%", pct)