Turn status card into a live stream with @micro AI mentions

[asim]

Users were already using the status card as chat — everyone posting updates that read like replies to each other. Instead of fighting it, make the stream the thing. The card is now a live scrollable feed of every status from everyone, and the AI is a first-class participant you can summon with @micro.

Data model:

• user.UpdateProfile now always pushes the previous status onto the history (not just on change), and merges the stored history with whatever the caller passed so a freshly-constructed *Profile{} can't accidentally drop history. History cap raised 20 → 100 per user.
• user.StatusStream(max) is new: flattens every user's current + history into one chronological feed, newest first, within the existing 7-day cutoff.

Home card:

• Replaced the "latest status per user" render with a shared RenderStatusStream helper used by both the home page and a new /user/status/stream fragment endpoint.
• The card body is wrapped in #home-status-wrap and followed by an inline <script> that polls /user/status/stream every 10 seconds, pauses when the tab is hidden, refreshes on focus, and preserves whatever the user is typing in the compose input.
• The compose form submits via fetch with the CSRF header, then refreshes the stream in place — no full page reload. Falls back to a native submit on network error.
• Removed the hover "clear status" button. Every post is appended to history, so clearing just trims the current entry and is functionally meaningless for a stream.
• Removed the "hide card when there's nothing" branch in favour of an empty-state line so the card is discoverable on new instances.

@micro AI participant:

• Posting a status containing @micro (word-bounded, so @microsoft and email@micro.xyz don't match) fires a background agent call whose answer is posted as a status from the existing app. SystemUserID ("micro") account. user.AIReplyHook is wired from main.go to agent.Query so the asker's wallet pays for the reply.
• user.PostSystemStatus posts as the system user without going through CanPost / CheckPostRate, and trims to MaxStatusLength.
• containsMention enforces word boundaries so mentions inside URLs or longer words don't trigger.

UX / mobile:

• MaxStatusLength raised 100 → 512 (tweet-ish).
• #home-statuses is capped at min(55vh, 420px) with overflow-y: auto and -webkit-overflow-scrolling so the card can never take the whole screen on mobile.
• .home-status-text now wraps properly (pre-wrap + overflow-wrap: anywhere) and the row uses align-items: flex-start so the avatar stays put when text wraps.
• Added a .home-status-system variant to tint @micro replies green.

Tests:

• user/status_test.go covers containsMention word boundaries, StatusStream chronological order across users and history, StatusStream max cap, and UpdateProfile always appending history (including repeat text).

The production /mu --serve deployment is unaffected — this touches handlers and renderers only, no startup or config changes.