docs(site): fix presenter-mode navigation on deployed deck

[jamesadevine]

Summary

Fixes presenter-mode navigation on the deployed intro deck. After the earlier routing fix (#1153), play mode worked but advancing slides in presenter mode duplicated the path segment:

…/ado-aw/slides/#/presenter/presenter/2

Root cause

Slidev builds every in-app navigation target with getSlidePath, which prepends the Vite base:

const path = presenter ? `presenter/${no}` : `${no}`
return `${import.meta.env.BASE_URL}${path}`

This couples the asset base and the router path to one value, so no single --base works for a sub-path deploy:

• --base /ado-aw/slides/ → base leaks into the route (#/ado-aw/slides/2) — breaks both modes.
• --base ./ → play works (#/2), but presenter pushes ./presenter/2, which Vue Router resolves relative to /presenter/1 → /presenter/presenter/2 — breaks presenter.

Fix

Decouple the two:

1. Build with the default base / — so BASE_URL stays / and hash routes resolve cleanly as /N and /presenter/N for both play and presenter mode.
2. vite.config.ts → experimental.renderBuiltUrl: { relative: true } — emits relative URLs for JS/CSS assets so the bundle loads from the sub-path.
3. postbuild.mjs — rewrites the entry HTML's remaining /assets/… references to ./assets/… (the one thing renderBuiltUrl doesn't cover).

routerMode: hash (already on main) is retained.

Test plan

Verified the built deck with a headless browser served at its real /ado-aw/slides/ base:

• Play: #/1 → advance → #/4 — no duplication, correct slide content.
• Presenter: #/presenter/1 → advance → #/presenter/4 — no duplication, renders.
• Deep-link reload at #/6 renders, including the Mermaid SVG diagram.
• Zero page errors.
• cd site && npm run build (the CI path: npm --prefix slides ci --omit=dev + slidev build + postbuild + astro build) completes; links validation passes; dist/slides/*.html ships relative ./assets/ references.