Server-side render login/logout nav based on session state

[Copilot]

Showing a Logout link to unauthenticated users is wrong. The previous flickering fix always rendered Logout for everyone since hiding/showing via JS was removed.

Approach

Render the correct nav-bottom HTML at the server level using auth.TrySession(r) — no JS toggling, no flicker.

• Template: Replaced hardcoded nav-bottom with a %s placeholder
• Two nav-bottom variants:
  • navBottomLogin — renders a Login link
  • navBottomAccount — renders username div + Account + Logout
• RenderHTMLForRequest: Checks session, injects the correct variant
• RenderHTML (no request context): Defaults to navBottomLogin
• Account handler: Upgraded from RenderHTML → RenderHTMLForRequest
• Mail handler: All three RenderHTML calls upgraded to RenderHTMLForRequest

func RenderHTMLForRequest(title, desc, html string, r *http.Request) string {
    lang := GetUserLanguage(r)
    _, acc := auth.TrySession(r)
    if acc != nil {
        return RenderHTMLWithLang(title, desc, html, lang, navBottomAccount)
    }
    return RenderHTMLWithLang(title, desc, html, lang, navBottomLogin)
}

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