Fix crash/hang on tab close + shutdown, and tutorial UI bugs (found in #416 review)

[VedMistry42]

Found these while reviewing #416, opening a fix branch off EY/MultiTabs directly since these bugs need to be resolved before that PR merges.

Crash/hang fixes:

• Closing a tab (or quitting the app) while it had an in-flight network request could force-kill a worker thread blocked in a network syscall, corrupting shared networking state and hanging all future model loads until restart
• Added local connection cancellation (RequestRegistry) so requests resolve promptly when a tab is closed/abandoned, and deferred tab destruction until any in-flight request has actually settled
• Tested extensively including repeated Cmd+Q-mid-process repros, multiple simultaneous pending tabs, and quitting while loading from the Home tab

Tutorial UI fixes:

• Steps checked a different tab accessor than the one actually used to load models, causing the tutorial to appear frozen and never advance
• Media Clipboard and Interface Summary steps failed to highlight anything (clipboard not shown by default; separately, an empty-rect painting bug drew a stray box)
• Spam-clicking Next created duplicate tabs (no guard against repeated calls while a load was in flight)
• Finishing the tutorial reset the auto-loaded tab instead of closing it, leaving it permanently blank
• One originally-reported issue (tutorial overlay covering the Home page) was intentionally left as-is after testing — preferred the original centered position

Not fixed here, flagged separately as out of scope (confirmed pre-existing via diff against develop, not introduced by #416):

• A crash in MidiDisplayComponent when loading a malformed MIDI file
• The Media Clipboard visibility default itself (separate from the tutorial highlight bug fixed above)

On the retry-crash bug (dangling pointer in ModelSelectionWidget): diagnosed once with a full stack trace early on, but extensively retested since — including under AddressSanitizer across multiple cycles — and unable to reproduce after the ModelTab.h changes in this branch. Not confirmed as an active blocker, noting here in case it resurfaces.

Tested manually throughout, plus an AddressSanitizer build for the retry-crash investigation.