fix: make Open in browser work from the first try.

[ValeraS]

🧢 Changes

Disable "Open in browser" menu item until branchUrl is evaluated.

☕️ Reasoning

branchUrl is Svelte's $direved value, that is lazy and not (re-)evaluated until it is actually read. branchUrl is only read inside the menu item’s click handler. On the first click, reading it initialises compareBranchUrlQuery, which starts the async forgeCompareBranchUrl request—but its response is still undefined and onClick handler does nothing.