Add support for tab encryption

[hluk]

This enables encrypting all tab data on disk that are not handled by plugins (i.e. Synchronization and Encryption which itself uses gpg) using single password.

The feature is disabled by default, needs to be enabled in build and in Preferences.

Requires QCA library.

To check if encryption is supported by the app (from Action dialog or command line):

copyq 'popup("Encryption", info("has-encryption") === "1" ? "Supported" : "NOT supported")'

TODO:

• Enable encryption in Linux builds
• Enable encryption in Windows builds
• Enable encryption in macOS builds
• Changing password
• Encrypting all data when enabled
• Disabling encryption - all the data need to be saved decrypted again
• Import and export with a custom password from GUI
• Support key stores (KWallet, GNOME Keyring, Credential Manager in Windows)
• Support encrypted import/export from scripts

Assisted-by: Claude Code

Fixes #2617

[codecov]

Codecov Report

❌ Patch coverage is 69.40981% with 368 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.47%. Comparing base (5bd157c) to head (34c80f2).

Files with missing lines	Patch %	Lines
src/common/encryption.cpp	52.30%	207 Missing ⚠️
src/gui/encryptionpassword.cpp	71.35%	53 Missing ⚠️
src/item/serialize.cpp	73.74%	47 Missing ⚠️
src/gui/mainwindow.cpp	58.53%	34 Missing ⚠️
src/gui/geometry.cpp	91.47%	11 Missing ⚠️
src/tests/itemtests/itemtests.cpp	68.18%	7 Missing ⚠️
src/tests/tests.cpp	61.53%	5 Missing ⚠️
src/common/config.cpp	0.00%	2 Missing ⚠️
src/gui/configurationmanager.cpp	92.85%	1 Missing ⚠️
src/item/itemfactory.cpp	92.85%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3345      +/-   ##
==========================================
- Coverage   79.93%   79.47%   -0.46%     
==========================================
  Files         231      236       +5     
  Lines       28469    29442     +973     
==========================================
+ Hits        22756    23400     +644     
- Misses       5713     6042     +329     

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