Handle hashtags

When adding hashtags to a post, they stay as static/passive text.
I imagine there's no easy way to render them as dynamic from the pod: what are they supposed to refer to?

[Example](https://masto.ai/@cybeardjm@mypod.store/113506466097137368)

No files or tests are named in the issue. Start by finding the post-composition/render path in the Mastopod frontend and locate where post body text is parsed before display. Reproduce with a sample post containing #tags (or the linked example) to confirm the current plain-text behavior, then trace how to make hashtag tokens render as interactive links. Done when a created post shows dynamic hashtag rendering and existing UI tests or manual checks confirm no regression.

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