sgf-viewer: Fix behavior of browser back button

This PR fixes issues with the browser back button.

Bug 1: Navbar in Firefox.

Repro: In Firefox (but not in Chrome), go to goattack.far.ai, click on "Baseline Attacks" in the navbar, and click the browser back button. The URL changes, but unexpectedly, the page content does not go back to the home page.

Bug: I'm not sure why this only happens in Firefox, but here's what I think is happening in App.svelte to cause this issue:

We have a variable let windowLocationPathname = window.location.pathname; that holds the current page path.
We have a reactive variable $: currentPath that determines the page contents. The variable indirectly depends on windowLocationPathname, so we expect changes in windowLocationPathname to propagate to currentPath.
In NavBar.svelte, when we click a link, we modify currentPath in order to change the page contents. Clicking the link also does change window.location.pathname, but this does NOT update windowLocationPathname.
When we click the browser's back button, we have a popstate handler that updates windowLocationPathname = window.location.pathname.
But since windowLocationPathname was never updated when we clicked a NavBar link, the value of windowLocationPathname is the same before and after the popstate handler. Then no changes are propagated to currentPath.

Fix: Make the popstate handler directly update currentPath.

Other fixes I considered:

Making windowLocationPathname reactive ($: windowLocationPathname = window.location.pathname;) doesn't work
SvelteKit has an import import { page } from '$app/stores';, and then you can get the current URL with $page.url.pathname. But SvelteKit is a framework based on Svelte, not a drop-in library interoperates with existing Svelte apps. It might not be easy to convert our existing webapp to SvelteKit.
svelte-router has a hook useLocation() that gets the current path, but in App.svelte it just returns undefined. I think it only tracks the location within svelte-router <Router> components?

Maybe there's an cleaner fix, I'm not familiar with Svelte

Bug 2: Table of contents (ToC)

Repro: Go to goattack.far.ai/transfer and alternately click on the two ToC entries a bunch of times to navigate between URL fragments #leela and #elf. Now click the URL back button a few times. The expected browser history is [transfer#leela, transfer#elf, transfer#leela, transfer#elf, ...]. But instead the history is [transfer#leela, transfer, transfer#elf, transfer, transfer#leela, transfer, transfer#elf, ...]

Fix: Just get rid of the onClick handler in the ToC that modifies the history with window.history.pushState(). I'm not sure why that onClick handler existed.

Testing

Tested the PR on Firefox and Chrome

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Latest commit	803434d92f384b1ee3fdb95d957e29d833bf109f
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Lighthouse	1 paths audited Performance: 90 (no change from production) Accessibility: 95 (no change from production) Best Practices: 100 (no change from production) SEO: 85 (no change from production) PWA: - View the detailed breakdown and full score reports

AlignmentResearch / KataGoVisualizer