fix: preview and export received attachments

[torlando-tech]

Summary

• make received image and file attachments tappable without changing their existing bubble styling or gestures
• present attachments with chat-owned SwiftUI Quick Look and preserve original bytes in isolated, safely named temporary items
• route multi-file messages by stable attachment index and refresh callbacks across timeline controller reuse
• add lifecycle, filename/type, accessibility, visual, static-contract, and Sideband-to-iOS regressions

Fixes #145

Verification

• xcodebuild test on the shipping Columba scheme: 278 tests, 0 failures
• focused attachment XTests: 9 tests, 0 failures
• Columba-ModelB simulator build: succeeded
• portable static suite: 244 tests passed, 1 skipped
• attachment preview contract: 3 runs, 41 assertions, 0 failures
• controlled Sideband interop: image, file, and second duplicate-name file flows all passed
  • tapped production attachment controls
  • opened native SwiftUI Quick Look
  • selected native Save Image / Save to Files
  • verified the active Quick Look export URL was byte-identical to the received attachment
• physical iPhone: exact-head signed build, install, and launch succeeded

Notes

On the iOS 26.4 Simulator, selecting Save to Files returns directly to Quick Look rather than presenting a second picker-level Save control. The interop lane therefore verifies selection of the named native action and byte identity at Quick Look's active export URL; it does not claim destination Files-container readback.

Two unrelated project-graph contracts remain baseline failures on the merged PR #149 baseline and are unchanged by this branch.

Risk and rollback

The change is scoped to attachment presentation and temporary export files. It does not alter LXMF encoding, persistence, database schema, or the messaging backend. Reverting this PR restores the previous noninteractive attachment rendering.

[greptile-apps]

Greptile Summary

The PR makes received image and file attachments interactive and presents their original bytes through native Quick Look.

• Adds isolated, sanitized temporary preview items with scoped cleanup.
• Routes image and indexed file-attachment actions through reusable timeline cells.
• Adds unit, visual, static-contract, and Sideband interoperability coverage.

Confidence Score: 5/5

The PR appears safe to merge.

No blocking failure remains.

Important Files Changed

Filename	Overview
Sources/ColumbaApp/Views/Messaging/MessageAttachmentPreviewItem.swift	Adds byte-preserving temporary preview items with filename sanitization, type detection, isolated ownership, and idempotent cleanup.
Sources/ColumbaApp/Views/Messaging/MessageBubble.swift	Converts rendered image and file attachments into accessible controls while retaining the bubble’s reaction gesture behavior.
Sources/ColumbaApp/Views/Messaging/MessageTimelineView.swift	Propagates refreshed attachment callbacks through the reusable UIKit timeline and validates file indexes before routing.
Sources/ColumbaApp/Views/Messaging/MessagingView.swift	Owns Quick Look presentation, prepares attachment URLs, reports preparation errors, and cleans preview resources on dismissal or conversation exit.
Tests/ColumbaAppTests/MessageAttachmentPreviewTests.swift	Covers byte preservation, filename safety, item lifecycle, callback routing, controller reuse, accessibility sizing, and visual rendering.
Tests/interop/test_attachments.py	Extends Sideband interoperability coverage through native preview and export flows for images and multiple files.
Columba.xcodeproj/project.pbxproj	Adds the preview implementation to both app flavors and registers its shipping-target tests.

Sequence Diagram

sequenceDiagram
    participant User
    participant Bubble as MessageBubble
    participant Timeline as MessageTimelineViewController
    participant Chat as MessagingView
    participant Item as PreviewItem
    participant QL as Quick Look
    User->>Bubble: Tap image or file chip
    Bubble->>Timeline: Attachment callback + stable index
    Timeline->>Chat: Open attachment
    Chat->>Item: Write original bytes to isolated temp URL
    Item-->>Chat: Preview URL
    Chat->>QL: Present URL
    QL-->>User: Preview and native export actions
    User->>QL: Dismiss
    QL->>Chat: Clear URL binding
    Chat->>Item: Clean owned temporary directory

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