docs: unreplied payment requests and last-comment filters

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Summary

Documents user-facing frontend changes from the past week around expense comment tracking and update emails.

Docs updated

• Submitting Expenses — new section on tracking payment requests, including the Unreplied tab on Incoming Money > Issued Payment Requests
• Paying Expenses as a Fiscal Host — revamped Last Comment By filter options and link to Overview Unreplied shortcut
• Fiscal Host Overview Dashboard — clarified what Unreplied means for expenses and applications in the To do list
• Managing Organization Finances — added Last Comment By filter guidance
• Grants — expanded Last Comment By filter options
• Updates and Contact — embedded videos in update emails now appear as linked preview images

Codepaths covered

• Frontend #12136 — Unreplied tab for outgoing/issued payment requests
• API #11724 — Last Comment By filter revamp (LastCommentBy enum)
• API #11954 — YouTube preview links in update notification emails

Knowledge gaps addressed

• Expense submitters had no documentation for finding requests awaiting their reply
• Host and org admins lacked guidance on the new Last Comment By filter values
• Contributors were not told how embedded videos render in update emails

Not documented (no user-facing change)

• Platform tips ledger v2 (#11924/#12280) — internal accounting only
• Private accounts visibility audit (#11936) — engineering hardening
• Dependency and i18n-only changes