feat: add code block copy button and popup markdown previewer with history

[harshvadhiya]

Add Code Block Copy Button & Popup Preview/History Features

Summary

This Pull Request introduces enhancements to improve the markdown rendering experience and add utility directly in the extension popup. It implements a copy button for rendered code blocks and adds a markdown editor tab inside the popup along with a preview history manager.

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Changes

1. Code Block Copy Button

• Appends a copy button in the top-right corner of all code blocks (excluding Mermaid diagrams) upon rendering.
• Added smooth transitions and color-coded success/failure states that adapt seamlessly to light and dark modes.
• Wired clipboard copying via navigator.clipboard.writeText with immediate visual feedback (displays checkmark ✔ for 2 seconds).

2. Popup Markdown Preview Tab

• Added a Preview tab in the extension popup featuring a markdown editor (textarea) and a Clear button to wipe input.
• Added a Preview button to render pasted Markdown. If the active tab is a new/empty tab page, it updates the tab directly to avoid creating duplicate preview pages.
• Paste data is preserved in local storage so that it does not disappear if the popup closes.

3. Preview History Tab

• Added a History tab in the popup containing a list of past preview items.
• Automatically assigns a title from the first heading of the Markdown (or fallback snippet) and records the preview timestamp.
• Actions:
  • Reopen: Open the past preview in a tab (updating the active tab if on a new-tab page).
  • Rename: Performs inline renaming of the entry by replacing the title label with an input box and Save (✔) / Discard (✖) controls.
  • Delete: Cleans up history entries and frees up extension local storage.
• Entry history list is capped at 50 items.

4. Compatibility & Performance Fixes

• Added "activeTab" permission to manifests to let the extension read active URLs for tab updates safely.
• Rerouted Prism.js syntax and MathJax extension scripts to load via direct script elements inside internal extension pages, preventing background worker crashes on extension tabs where sender.tab is undefined.

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Files Modified / Created

• manifest.json, manifest.chrome.json, manifest.firefox.json - Added activeTab permission.
• content/index.css - Styled copy buttons, layouts, and theme support.
• content/index.js - Added copy button logic on rendering.
• content/prism.js / content/mathjax.js - Dynamic script loading logic for internal extension pages.
• content/preview.html / content/preview.js - [NEW] Setup the rendering template for preview pages.
• popup/index.css - Formatted popup tabs, textarea grid, preview buttons, and history list.
• popup/index.js - Added history/editing trackers, reopen tabs checks, inline editor renders, and storage binds.
• background/messages.js - Guarded message handling routes.

Sample Images