feat(collections): add real-time updates via WebSocket (Limitation #4)

Implement real-time collection updates when books are added/removed:

Backend Changes:
- Added connManager to CollectionHandler struct
- Updated constructor to accept ConnectionManager
- Updated all NewCollectionHandler() calls in ebook.go and main.go
- Added WebSocket broadcasts in AddBooks() handler
- Added WebSocket broadcasts in BulkRemoveBooks() handler
- Broadcasts collection_updated events with:
  - collection_id: Which collection changed
  - action: books_added or books_removed
  - book_ids: Array of affected book IDs
  - count: Number of books changed

Frontend Changes:
- Added WebSocket connection in collections UI
- connectWebSocket() establishes connection to /ws/sync
- Listens for collection_updated events
- Shows toast notification on collection change
- Auto-reloads page after 1 second to show updated book list
- Auto-reconnect on disconnect (5s delay)
- Error handling for WebSocket failures

WebSocket Event Format:
{
  "type": "collection_updated",
  "timestamp": "2026-02-01T12:00:00Z",
  "data": {
    "collection_id": "uuid",
    "action": "books_added",
    "book_ids": ["uuid1", "uuid2"],
    "count": 2
  }
}

User Experience:
- When another user adds books to a collection, all connected clients see:
  1. Toast notification: "Collection updated: books_added (2 books)"
  2. Page auto-refreshes after 1 second
  3. Updated book list displays
- Same for book removal
- Works across multiple browser tabs/devices
- No manual refresh needed

Technical Notes:
- Broadcasts to ALL connected WebSocket clients
- Client-side filtering by collection_id
- Existing progress/conflict broadcasts continue to work
- Connection manager handles broadcast distribution

Resolves Limitation #4: Real-time Collection Updates

internal/handlers/collections.go

@@ -3,11 +3,13 @@ package handlers
 import (
 	"bookmann/internal/database"
 	"bookmann/internal/services"
+	wsync "bookmann/internal/sync"
 	"encoding/json"
 	"fmt"
 	"net/http"
 	"strconv"
 	"strings"
+	"time"
 
 	"github.com/google/uuid"
 	"github.com/jackc/pgx/v5/pgtype"
@@ -17,12 +19,14 @@ import (
 type CollectionHandler struct {
 	db                *database.Queries
 	collectionService *services.CollectionService
+	connManager       *wsync.ConnectionManager
 }
 
-func NewCollectionHandler(db *database.Queries) *CollectionHandler {
+func NewCollectionHandler(db *database.Queries, connManager *wsync.ConnectionManager) *CollectionHandler {
 	return &CollectionHandler{
 		db:                db,
 		collectionService: services.NewCollectionService(db),
+		connManager:       connManager,
 	}
 }
 
@@ -276,12 +280,31 @@ func (h *CollectionHandler) AddBooks(c echo.Context) error {
 		return c.JSON(http.StatusBadRequest, map[string]string{"error": err.Error()})
 	}
 
+	addedCount := 0
+	var addedBookIDs []string
 	for _, bookIDStr := range req.BookIDs {
 		bookID, err := uuid.Parse(bookIDStr)
 		if err != nil {
 			continue
 		}
-		h.collectionService.AddBookToCollection(c.Request().Context(), collectionID, bookID, userUUID)
+		err = h.collectionService.AddBookToCollection(c.Request().Context(), collectionID, bookID, userUUID)
+		if err == nil {
+			addedCount++
+			addedBookIDs = append(addedBookIDs, bookID.String())
+		}
+	}
+
+	if addedCount > 0 && h.connManager != nil {
+		h.connManager.Broadcast(wsync.BroadcastMessage{
+			Type:      "collection_updated",
+			Timestamp: time.Now().Format(time.RFC3339),
+			Data: map[string]interface{}{
+				"collection_id": collectionID.String(),
+				"action":        "books_added",
+				"book_ids":      addedBookIDs,
+				"count":         addedCount,
+			},
+		})
 	}
 
 	return c.NoContent(http.StatusNoContent)
@@ -325,6 +348,7 @@ func (h *CollectionHandler) BulkRemoveBooks(c echo.Context) error {
 	}
 
 	removedCount := 0
+	var removedBookIDs []string
 	for _, bookIDStr := range req.BookIDs {
 		bookID, err := uuid.Parse(bookIDStr)
 		if err != nil {
@@ -334,9 +358,23 @@ func (h *CollectionHandler) BulkRemoveBooks(c echo.Context) error {
 		err = h.collectionService.RemoveBookFromCollection(c.Request().Context(), collectionID, bookID)
 		if err == nil {
 			removedCount++
+			removedBookIDs = append(removedBookIDs, bookID.String())
 		}
 	}
 
+	if removedCount > 0 && h.connManager != nil {
+		h.connManager.Broadcast(wsync.BroadcastMessage{
+			Type:      "collection_updated",
+			Timestamp: time.Now().Format(time.RFC3339),
+			Data: map[string]interface{}{
+				"collection_id": collectionID.String(),
+				"action":        "books_removed",
+				"book_ids":      removedBookIDs,
+				"count":         removedCount,
+			},
+		})
+	}
+
 	return c.JSON(http.StatusOK, map[string]interface{}{
 		"removed": removedCount,
 		"total":   len(req.BookIDs),