Fix blog tag filter selected state and context

[NIRMAL20062]

Description

Tag filter pills were missing entirely on tag pages (e.g. /blog/tags/baking/).
The "All" pill was always shown as selected even when filtering by a tag,
and the active tag had no visual highlight.

Fixes:

• Tag pills now render on tag-filtered pages
• "All" is selected on /blog/, becomes a link on tag pages
• Active tag pill is highlighted when filtering
• Removed redundant aria-label on tag links (visible text is sufficient)
• Fixed template variable shadowing (tag → child_tag in loop)

AI usage

Used GitHub Copilot for implementation support and verification and refinement of the description.

thanks.