feat(message): copy and save message images and link actions in the c…

[Th-Underscore]

Description

Context-menu additions for messages:

• Copy Image / Save Image on right-clicking any image in a message: encrypted attachments use the decrypt-aware media download; other images (avatars, URL-preview thumbnails, inline) use their displayed source with format-aware handling. "Save Image" reuses the zoom-in viewer's save path with smarter filenames - the image's own title/alt is preferred, then the decoded URL basename, then a generic name, always sanitized and given the correct extension from the actual content type. Both close immediately with a busy cursor plus success/failure toast, mimicking Discord. If requested, these flairs can be toned down/removed for the sake of simplicity.
• Copy URL / Open Link on right-clicking links. "Open Link" mirrors a real target="_blank" anchor click as window.open doesn't route to the system browser from Sable's desktop webview.

The zoom-in viewer's own Save As picks up the same filename/extension improvements, keeping both paths consistent.

Tested on the desktop app, Ubuntu 24.04.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation - .changeset
• My changes generate no new warnings

AI disclosure:

• Partially AI assisted (clarify which code was AI assisted and briefly explain what it does).
• Fully AI generated (explain what all the generated code does in moderate detail).

I designed and directed the features and tested them in the GUI; the code itself was AI-generated.

Copy/Save reuse the existing zoom-in primitives (copyImageToClipboard, downloadMedia, saveFileToDevice) factored into small tested pure helpers (downloadMessageImage, resolveImageSaveFilename). Image detection is closest('[data-message-attachment], img'), so any image works while encrypted attachments keep their decrypt path. Filename resolution lives in $utils/download.ts alongside the existing sanitizer, with MIME-aware extensions derived from the downloaded blob's actual type instead of possibly-absent metadata. Clipboard failures throw so the caller can toast instead of failing silently.