feat(extension): restrict attachments to images only and add DeepSeek V4 models

[lcomplete]

Summary

• Image-only attachments: Restrict file attachments in the sidepanel composer to images only (validated by MIME type with extension-based fallback). Updates drag-and-drop, clipboard paste, and file picker to silently ignore non-image files. Changes UI copy from "files/attachments" → "images" throughout.
• DeepSeek V4 models: Add deepseek-v4-flash and deepseek-v4-pro preset models to the provider registry while preserving existing defaults.
• CI pipeline improvements: Add typecheck step, switch from yarn build to yarn zip, add Python verification script that checks manifest version matches tag version and zip contains manifest.json.
• Version bump: Extension version 1.0.0 → 0.5.5.
• TypeScript fixes: Improve types for chrome.scripting dynamic API, context menu context arrays, and message response casting. Clean up MessageFooter function overload.

Test plan

• Drag an image file into the sidepanel — should attach successfully
• Drag a PDF or other non-image file — should be ignored silently
• Paste an image from clipboard — should attach
• Paste a non-image file — should not attach
• Open action menu in composer — icon is ImagePlus, label is "Add images"
• File picker opened via action menu accepts only image/*
• DeepSeek provider shows V4 Flash and V4 Pro model options
• Extension CI workflow passes with typecheck and zip verification steps

🤖 Generated with Claude Code

[augmentcode]

🤖 Augment PR Summary

Summary: This PR tightens the browser extension’s attachment handling to image-only inputs, adds new DeepSeek V4 preset models, and strengthens the extension release CI.

Changes:

• Restricts composer attachments to images across drag-and-drop, clipboard paste, and the file picker (MIME validation with extension-based fallback) and updates UI copy from “files/attachments” to “images”.
• Adds deepseek-v4-flash and deepseek-v4-pro to the DeepSeek provider registry while keeping existing defaults first.
• Improves extension typing around dynamic chrome.scripting APIs, context-menu context arrays, and sidepanel context-menu message responses; adds a global chrome types reference.
• Updates the release workflow to use yarn install --frozen-lockfile, run yarn typecheck, package via yarn zip, and verify manifest/version + zip contents.
• Aligns manifest versioning to EXTENSION_VERSION / npm_package_version and sets the extension package version to 0.5.5.

Technical notes: Non-image drops/pastes are silently ignored; image detection uses file.type first with a filename-extension fallback map.

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[augmentcode]

Review completed. 3 suggestions posted.

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[augmentcode]

app/extension/package.json:3 The extension version is being set from 1.0.0 down to 0.5.5; can you confirm this won’t break update/install flows (Chrome Web Store requires monotonically increasing versions once a build is published)?

Severity: medium

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[augmentcode]

app/extension/src/sidepanel/hooks/useDragAndDropZone.ts:32 The fallback : true when item.type is empty will treat unknown file drags as “image droppable”, so the “Drop images” overlay may show for non-image files that then get ignored on drop.

Severity: low

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[augmentcode]

app/extension/src/sidepanel/utils/tabContext.ts:158 Since mediaType can now be inferred from the file extension, readFileAsDataUrl(file) may still produce a data: URL with a non-image/* MIME when file.type is empty, which could make <img src={dataUrl}> previews fail despite mediaType indicating an image.

Severity: medium

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