fix(pages): DocsPage does not scroll to in-page anchor on initial load / deep link

[lizhengfeng101]

Description

DocsPage handles in-page anchor scrolling only on click (via handleContentClick and scrollToHeading). It never reads location.hash when the page first mounts, so deep-linking to a heading does not scroll to it.

Open a URL such as .../#/docs/faq#超时 (or .../#/docs/configuration#timeouts) directly in a fresh tab: the correct doc renders, but the viewport stays at the top instead of scrolling to the 超时 / Timeouts heading. Clicking the same link from within the app works, because that path goes through the click handler.

Note the site uses HashRouter (pages/src/index.tsx), so the routed URL is #/docs/faq#超时 — react-router parses this into pathname: "/docs/faq" and hash: "#超时". A fix should read the fragment from useLocation().hash, not from window.location.hash directly.

Current behavior:

• Click an in-page link inside the app → scrolls ✅ (handleContentClick, DocsPage.tsx:199-233)
• Load/refresh a URL that already contains a heading fragment → no scroll ❌ (nothing reads the hash on mount)

Expected behavior:

• On initial mount (and whenever location.hash changes), scroll the matching heading into view, mirroring the existing click behavior.

Scope

• File: pages/src/pages/DocsPage.tsx
• Area: add an effect that runs after the doc content renders, reads useLocation().hash, decodes it, and scrolls the target heading into view.
• Reuse the existing decodeFragment helper (already in DocsPage.tsx) — marked percent-encodes non-ASCII fragments while heading ids are raw text from generateHeadingId, so the fragment must be decoded before getElementById.
• Headings are rendered asynchronously from markdown, so the lookup likely needs the same requestAnimationFrame retry pattern already used in the cross-page anchor path (DocsPage.tsx:222-230).

Acceptance Criteria

• Loading .../#/docs/faq#超时 (zh), .../#/docs/faq#no-tool-calls-parsed-local-models-ollama (en), and .../#/docs/configuration#timeouts directly scrolls to the target heading.
• Works for CJK fragments (decoded via decodeFragment).
• Existing in-app click-to-scroll behavior is unchanged.
• No scroll/console error when the fragment is empty or does not match any heading.
• Code check passes (make check)

Context

Discovered during review of #400, which added CJK cross-links to the docs and fixed click-time anchor decoding (decodeFragment). The mount-time / deep-link case was explicitly noted as out of scope there and is filed here as a follow-up. The decodeFragment helper introduced by #400 should be reused for consistency.

[fjbarrett]

I would like to work on this. I will add mount/hash-change scrolling that reuses the existing fragment decoder and retry behavior, cover the direct-link cases, and verify the existing click flow remains unchanged.