Add lyrics spellcheck feature (Tools > Check lyrics spelling...)

[blaskocode]

Resolves: Feature request for lyrics spellchecking in MuseScore

Summary

This PR adds a new Tools > Check lyrics spelling... feature that allows users to scan all lyrics in their score for spelling errors using the Hunspell spell checking library.

Problem Solved

Lyrics entry in MuseScore is error-prone. The current workaround requires:

1. Tools → Copy lyrics to clipboard
2. Paste into a word processor
3. Spellcheck externally
4. Manually fix typos back in MuseScore

This is slow and brittle, especially on large scores or multi-verse choral works.

Feature Overview

• Menu action: Tools > Check lyrics spelling...
• Dialog: Shows unknown words with count and first location (measure, staff, verse)
• Navigation: Click "Go to" or double-click to navigate to the lyric in the score
• Rescan: Re-run spellcheck after making corrections
• Language: Auto-detects from system locale with platform-specific dictionary discovery

Architecture

This is an end-to-end feature spanning:

Layer	Components
Build System	CMake Hunspell detection via pkg-config
Backend	HunspellSpellChecker with platform-specific dictionary discovery
Service	LyricsSpellCheckService with word reconstruction from syllables
Model	LyricsSpellingIssuesModel (QAbstractListModel)
UI	QML dialog following existing patterns

Files Added (22 files, ~1,500 lines)

• Interfaces: ispellchecker.h, ilyricsspellcheckservice.h
• Implementation: hunspellspellchecker.cpp/.h, lyricsspellcheckservice.cpp/.h, stubspellchecker.h
• UI: LyricsSpellingIssuesDialog.qml, lyricsspellingissuesmodel.cpp/.h
• Build: SetupHunspell.cmake, CMakeLists changes
• Tests: lyricsspellcheck_tests.cpp, spellcheckermock.h
• Integration: Action controller, UI actions, menu model changes

Test Plan

• Build succeeds with Hunspell (cmake -DMUE_COMPILE_USE_SYSTEM_HUNSPELL=ON)
• Build succeeds without Hunspell (stub implementation used)
• Menu item appears under Tools menu when score is open
• Warning shown when no dictionaries installed
• Dialog correctly lists misspelled words from test scores
• "Go to" navigates to and selects the correct Lyrics element
• Score viewport scrolls to show the selected lyric
• "Rescan" updates list after fixing typos
• Empty state shows "All lyrics are spelled correctly!" when no issues
• Dialog can remain open while editing score
• Multi-verse/multi-staff scores handled correctly
• Hyphenated lyrics reconstructed as words

Prerequisites for Testing

macOS: brew install hunspell
Linux: sudo apt install hunspell libhunspell-dev hunspell-en-us
Windows: Install hunspell and place dictionaries in %APPDATA%/hunspell/

Checklist

• I signed the CLA
• The title of the PR describes the problem it addresses
• Each commit's message describes its purpose and effects
• If changes are extensive, there is a sequence of easily reviewable commits
• The code in the PR follows the coding rules
• There are no unnecessary changes
• The code compiles and runs on my machine
• I created a unit test to verify the changes I made