feat: enhance PDF reading and document editing

[naveensingh]

Type of change(s)

• Bug fix
• Feature / enhancement
• Infrastructure / tooling (CI, build, deps, tests)
• Documentation

What changed and why

• PDF text selection and copying
• Option for dark PDF pages
• Option for horizontal PDF scrolling
• PDF text search
• PDF table of contents
• Option to hide floating PDF page numbers
• Reopen to edit button for read-only files
• Support for editing more text formats
• Fixed files opened from other apps missing from Recent apps
• Fixed CSV editing for files opened from other apps
• Edit a copy and Save a copy options
• Removed leading icon indicators from overflow menu items

Tests performed

Tested PDF search, selection, copying, external intents, dark mode, zoom, rotation, etc.

Closes the following issue(s)

• Closes No option to edit CSV files #70
• Closes Files open as read-only #71
• Closes Documents disappears after switching away from an opened PDF or VCF #73
• Closes correctly recognize MIME type text/org-mode #76
• Closes Chapters navigation #77
• Closes Option to turn off floating page numbers #79
• Closes search for text #87

Checklist

• I read the contribution guidelines.
• I manually tested my changes on device/emulator (if applicable).
• I updated the "Unreleased" section in CHANGELOG.md (if applicable).
• I have self-reviewed my pull request (no typos, formatting errors, etc.).
• I understand every change in this pull request.