fix(MessageBubble): clamp future-dated timestamps to now

[torlando-agent]

Implements the iOS half of the approved plan for #60.

Summary

LXMF wire format puts a single timestamp on each message: the sender's time.time() at pack time (set in LXMessage.swift:171-179, matching RNS/LXMF/LXMessage.py:362). Columba-iOS treats that field as the canonical timestamp for both display and ordering — see MessageBubble.swift:329, 384 for display, and LXMFDatabase.swift:447, 213 for the SQL ORDER BY timestamp DESC that drives the chat ScrollView. Result: any clock skew between two devices (or any clock drift on the local device) leaks straight into the UI as out-of-order messages and "in 5 min" relative-time strings. Both observed symptoms collapse to that one cause.

The fix has two halves: (1) sort the chat scroll by local arrival time (created_at, already populated on every save in MessageRecord.swift:182-184), so a peer with a stale or fast clock can't reorder my own thread; (2) clamp displayed relative time so a future-dated wire timestamp renders as "Just now" instead of "in 5 min".

This PR is half (2) — the iOS clamp. Half (1) lives in LXMF-swift and ships as a separate PR per the plan's split.

Changes

• Sources/ColumbaApp/Views/Messaging/MessageBubble.swift — clamp Message.formattedTime to min(timestamp, Date()) before handing to RelativeDateTimeFormatter. The bubble's absolute-time formatter at MessageDetailView.swift:527 is intentionally left alone — that view shows raw protocol-level metadata.
• Tests/ColumbaAppTests/MessageFormattedTimeTests.swift — new test file with two cases: future-dated timestamps render as the formatter's "now" string; past-dated timestamps still render the formatter's relative-past string.
• Columba.xcodeproj/project.pbxproj — wire the new test file into the ColumbaAppTests target (PBXBuildFile / PBXFileReference / group / sources phase).

Test plan

• test_formattedTime_clampsFutureTimestampToNow — constructs a 600 s future-dated Message, asserts formattedTime == formatter.localizedString(for: now, relativeTo: now). Captures whatever the formatter actually emits rather than hard-coding "Just now" so locale variation doesn't break it.
• test_formattedTime_pastTimestampsRenderRelativePast — regression guard that the clamp doesn't affect past timestamps.
• Full suite: xcodebuild test -only-testing:ColumbaAppTests — 67 passed, 0 failed.
• xcodebuild build clean (warnings present in the build log are pre-existing Swift 6 sendability warnings unrelated to this change).

Implementer notes

• The full issue resolution depends on a companion LXMF-swift PR that swaps the chat-scroll ordering from wire timestamp to created_at. This iOS PR alone covers the "in 5 min" symptom only; the out-of-order-messages symptom needs the LXMF-swift PR. Commit message uses Refs #60 (not Closes) for that reason.
• No Package.swift version bump for LXMF-swift in this PR — the LXMF-swift change isn't released yet, so bumping the pin would break SPM resolution. The bump should land in the LXMF-swift PR's follow-up or a coordination PR once the LXMF-swift release is tagged.

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