fix: wire up file picker change handler for attach button

[PureWeen]

Problem

The attach image button (📎) does nothing on mobile. Tapping it opens the native file picker, but selecting an image has no effect — the image is silently dropped.

Root Cause

The button clicks a hidden <input type="file"> via clickElement() JS interop, which correctly opens the native picker. But there is no change event listener on the file input, so when the user selects a file, nothing reads it.

On desktop, image attachment works via paste and drag-drop, which have their own global JS listeners. The file picker button has never actually worked on any platform — it's just more noticeable on mobile where paste/drag-drop aren't available.

Fix

Add a global delegated change event listener for <input type="file"> elements matching the file-{sessionName} pattern. When files are selected:

1. Read each image file via FileReader
2. Call JsImagePasted (the same Blazor interop used by paste and drag-drop)
3. Reset the input value so the same file can be re-selected

This follows the exact same pattern as the existing paste and drop handlers.

Testing

• ✅ All 3319 tests pass
• ✅ Mac Catalyst build succeeds
• Needs manual verification on Android/iOS that the native file picker → image attachment flow works end-to-end

[PureWeen]

PR #558 Multi-Model Code Review (Re-Review v3 — Final)

CI status: ⚠️ no checks reported on this branch.
Verified manually: ✅ Image attachment tested end-to-end on Android and iOS.

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Previous Findings

#	Finding	Status
1	🔴 Orphaned dragover listener breaking all JS	✅ FIXED
2	🟢 Temp files never cleaned up (2/3)	✅ FIXED — finally block deletes temp files after dispatch

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Current Review (3/3 reviewers completed)

All reviewers confirmed the core flow is correct:

• ✅ JS file-picker change handler — proper IIFE closure, input reset
• ✅ MediaPicker on mobile — native photo picker, streams properly disposed
• ✅ Base64 encoding/decoding — File.Exists guard, try/catch per file, GUID filenames prevent path traversal
• ✅ Image-only message validation — allows empty text when images present
• ✅ 16MB message size limit — necessary for base64 images, reasonable cap
• ✅ Interface change — backwards compatible with default null parameter
• ✅ Fire-and-forget closure — sendImagePaths captured by value, no shared mutation race
• ✅ Temp file cleanup — finally block deletes decoded images after dispatch

Informational (single-reviewer, not actionable)

• Images dropped during restore queue: PendingBridgePrompt stores only session/message/mode — images lost if user sends during the ~2s startup restore window. Narrow edge case, follow-up. (1/3)

Test Coverage

• JS interop layer has no automated tests (pre-existing)
• Bridge protocol changes verified via manual end-to-end testing on Android + iOS
• Test stub updated for new interface signature ✅

Recommendation

✅ Approve — all findings addressed. Verified end-to-end on both platforms. Ready to merge.