web: apply frontend review fixes

[jpenilla]

Applies frontend review feedback for the web interface.

Auth & routing

• Replace onMount-based auth redirects with prerender-safe +page.ts load guards:
  / → /login when no token, /login → / when a token exists. Removes the
  flash of the dashboard before redirect.
• Drop the redundant token check from login/+page.svelte.

API & state

• Simplify fetchApi: relative URLs, drop dead headers spread, correctly merge
  Authorization with caller-provided headers, and fix query-string construction.
• Use the reactive token state instead of direct localStorage reads.
• URL-encode the Minecraft version in API requests.

Components & accessibility

• Fix PatchesStats stuck on "Loading statistics…" when total === 0.
• Fix duplicated dark:dark:bg-white/20 Tailwind class.
• Add a11y: role="progressbar" + ARIA values on the stats bar, aria-pressed
  on the Table/Stats toggles, and autocomplete on the login inputs.
• Clean up LabeledCheckbox, SettingsPopoverGroup, PatchesTable, and
  theme.svelte.ts (dead snippets/script/style blocks, unused snippet params,
  SSR-safety note).

Notes

• getUsername() keeps its original throw-on-missing-token behavior; the
  dashboard guards display with a ternary.
• Verified with bun run check, bun run lint, and bun run build, plus a
  manual end-to-end pass against a local backend (login, dashboard, stats,
  logout redirects).