fix: virtual scroll jumping with images/GIFs

Server:
- Extract image width/height on upload via image.DecodeConfig (header-only)
- Add width/height columns to attachments table (migration 007)
- Include dimensions in AttachmentInfo JSON (optional, backward-compatible)

Client:
- Reserve exact space for images using server-provided dimensions
- Fallback min-height: 200px for external/old images, cleared on load
- Remove premeasureAll() — was caching wrong heights for unloaded images
- Smart per-type height estimates: 32px dividers, 42/72px text, +220px
  per image attachment, +320px for YouTube embeds
- Batch ResizeObserver corrections to single RAF with anchor-based scroll
  preservation (topmost visible item stays in place)
- Fenwick tree for O(log n) offset lookups (replaces O(n) linear scans)
- CSS contain: layout style on .msg-image to isolate layout shift

Server/db/attachment_queries.go

@@ -19,10 +19,11 @@ type Attachment struct {
 }
 
 // CreateAttachment inserts a new attachment record (initially unlinked to any message).
-func (d *DB) CreateAttachment(id, filename, storedAs, mimeType string, size int64) error {
+// width and height are optional image dimensions (pass nil for non-image files).
+func (d *DB) CreateAttachment(id, filename, storedAs, mimeType string, size int64, width, height *int) error {
 	_, err := d.sqlDB.Exec(
-		`INSERT INTO attachments (id, filename, stored_as, mime_type, size) VALUES (?, ?, ?, ?, ?)`,
-		id, filename, storedAs, mimeType, size,
+		`INSERT INTO attachments (id, filename, stored_as, mime_type, size, width, height) VALUES (?, ?, ?, ?, ?, ?, ?)`,
+		id, filename, storedAs, mimeType, size, width, height,
 	)
 	if err != nil {
 		return fmt.Errorf("CreateAttachment: %w", err)
@@ -88,7 +89,7 @@ func (d *DB) GetAttachmentsByMessageIDs(msgIDs []int64) (map[int64][]AttachmentI
 	}
 
 	query := fmt.Sprintf(
-		`SELECT id, message_id, filename, size, mime_type
+		`SELECT id, message_id, filename, size, mime_type, width, height
 		 FROM attachments WHERE message_id IN (%s)`,
 		strings.Join(placeholders, ","),
 	)
@@ -103,7 +104,7 @@ func (d *DB) GetAttachmentsByMessageIDs(msgIDs []int64) (map[int64][]AttachmentI
 		var id string
 		var msgID int64
 		var ai AttachmentInfo
-		if scanErr := rows.Scan(&id, &msgID, &ai.Filename, &ai.Size, &ai.Mime); scanErr != nil {
+		if scanErr := rows.Scan(&id, &msgID, &ai.Filename, &ai.Size, &ai.Mime, &ai.Width, &ai.Height); scanErr != nil {
 			return nil, fmt.Errorf("GetAttachmentsByMessageIDs scan: %w", scanErr)
 		}
 		ai.ID = id

Server/db/models.go

@@ -135,6 +135,8 @@ type AttachmentInfo struct {
 	Size     int64  `json:"size"`
 	Mime     string `json:"mime"`
 	URL      string `json:"url"`
+	Width    *int   `json:"width,omitempty"`
+	Height   *int   `json:"height,omitempty"`
 }
 
 // ReactionInfo is the reaction shape in API responses.