Paste as plain text (#14)

[mobrava]

Refs #14 (kept open until this ships in a release)

What

• PasteService.paste(item:asPlainText:): passing nil resolves the mode from the setting XOR the Shift modifier.
• Card context menu is split into Paste with Formatting / Paste as Plain Text, shown only for RTF and HTML clips.
• New Always Paste as Plain Text toggle in Settings > General (off by default).

Holding Shift while pasting flips the configured behavior for that single paste. Existing paste(item:) call sites (Return key, card click, preview, menu bar) keep their signature and go through the modifier-aware path automatically.

Deliberately out of scope

• Version bump, appcast.xml, tag, and release: 1.3.0 (23) is currently in App Store review, so shipping is deferred until review clears and the GitHub release is out.
• README feature list: will be updated together with the release.
• Whitespace/line-break cleanup, encoding repair, and OCR (things PastePlain also does) are not part of this issue.

Verification status

• swiftc -parse passes on the changed files.
• Full build and E2E are not done yet. They will be run locally before release:

  DEVELOPER_DIR=/Applications/Xcode.app/Contents/Developer xcodebuild -project Clipbara.xcodeproj -scheme Clipbara -configuration Debug build
  

  Checks: (1) copy formatted web text, Shift+click a card, and confirm only plain text lands on the clipboard, (2) a normal click still keeps formatting, (3) turning the setting on inverts both behaviors.