feat(sd): SD-11467 Fix misc. crashes

[parthshahp]

What?

This PR fixes three performance/stability regressions, all related to SD-11467:

1. Restore early onThemeBundleMain execution (templates/layout/base.html)

Fixed by checking typeof window.stencilBootstrap === 'function' (set synchronously when the bundle executes). If already loaded, call onThemeBundleMain() immediately; otherwise attach the load listener.

2. Guard against null response in optionChangeDecorator (product-details-base.js)

Added if (err) return; guard before accessing response.data/response.content to prevent a secondary TypeError masking the real error.

3. Fix updateWalletButtonsView crash on cart edit modal (product-details-base.js)

Used optional chaining (?.[0]?.checkValidity() ?? true) to handle the cart edit modal where no Add To Cart form exists, fixing a crash introduced in PR #2532.

Requirements

• CHANGELOG.md entry added (required for code changes only)

Tickets / Documentation

• SD-11467
• PR #2536 (introduced regression 1)
• PR #2532 (introduced regression 3)

Screenshots (if appropriate)

N/A — all fixes are JS/HTML logic with no visual changes.