Fix potential crash in asset viewer

[samhenrigold]

Fixes a potential crash caused by unsafe reference in scroll notification handler. Changed [unowned self] to [weak self] and added proper cleanup of notification observer in deinit to prevent accessing deallocated view controller.

Crash log that sparked this:

-------------------------------------
Translated Report (Full Report Below)
-------------------------------------

Process:               Samra [2744]
Path:                  /Users/USER/Downloads/Samra.app/Contents/MacOS/Samra
Identifier:            com.serena.Samra
Version:               1 (1.4)
Code Type:             ARM-64 (Native)
Parent Process:        launchd [1]
User ID:               501

Date/Time:             2024-12-08 17:32:27.6506 -0500
OS Version:            macOS 15.1.1 (24B91)
Report Version:        12
Anonymous UUID:        1ED1F224-511F-08C2-995E-C72747367227

Sleep/Wake UUID:       735F8F37-5179-4987-BE1D-7BF100AD7D84

Time Awake Since Boot: 78000 seconds
Time Since Wake:       6867 seconds

System Integrity Protection: disabled

Crashed Thread:        0  Dispatch queue: com.apple.main-thread

Exception Type:        EXC_CRASH (SIGABRT)
Exception Codes:       0x0000000000000000, 0x0000000000000000

Termination Reason:    Namespace SIGNAL, Code 6 Abort trap: 6
Terminating Process:   Samra [2744]

Application Specific Information:
abort() called


Thread 0 Crashed::  Dispatch queue: com.apple.main-thread
0   libsystem_kernel.dylib        	       0x1954a6600 __pthread_kill + 8
1   libsystem_pthread.dylib       	       0x1954def70 pthread_kill + 288
2   libsystem_c.dylib             	       0x1953eb908 abort + 128
3   libswiftCore.dylib            	       0x1a6509e6c swift::fatalErrorv(unsigned int, char const*, char*) + 136
4   libswiftCore.dylib            	       0x1a6509e8c swift::fatalError(unsigned int, char const*, ...) + 32
5   libswiftCore.dylib            	       0x1a650a050 swift::swift_abortRetainUnowned(void const*) + 32
6   libswiftCore.dylib            	       0x1a6579124 swift_unknownObjectUnownedLoadStrong + 76
7   Samra.debug.dylib             	       0x102da7b8c closure #4 in RenditionListViewController.loadView() + 252
8   Samra.debug.dylib             	       0x102da84f8 thunk for @escaping @callee_guaranteed @Sendable (@in_guaranteed Notification) -> () + 160