fix: reject invalid characters in hashtag input field

[barrydeen]

Summary

• Filter hashtag input as-you-type to only allow letters, digits, and underscores
• Strip a leading # if typed (users naturally type #bitcoin)
• Remove now-redundant .trim().removePrefix("#") from submit handler and simplify enabled check

Prevents a crash where a user entering #hashtag1, #hashtag2 would store a comma-containing value that breaks the comma-delimited hashtag_set navigation route.

Test plan

• Type #hashtag1, #hashtag2 in the add-hashtag field — only hashtag1 should appear (comma and everything after is dropped)
• Type #bitcoin — should appear as bitcoin (leading # stripped)
• Type a hashtag with spaces — spaces should not be entered
• Normal hashtag entry and submission still works

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