Add targeted practice and custom text practice modes

[ltraj]

Summary

This PR adds two new practice modes to FingerGo.

Targeted Practice

Users can generate exercises focused on specific key groups:

• Home Row
• Left Hand
• Right Hand
• Top Row
• Bottom Row
• Numbers
• Punctuation
• Symbols
• Weak Keys
• Custom Groups

Features:

• Group selection UI
• Exercise preview generation
• Session metadata support
• Weak key recommendations

Custom Text Practice

Users can:

• Paste arbitrary text
• Preview before starting
• Practice immediately
• Optionally save later

Examples:

• Resume practice
• Interview answers
• Articles
• Notes
• Custom typing exercises

Session Metadata

Added support for:

• PracticeMode
• PracticeGroupId
• PracticeGroupName
• TargetKeys

Testing

Tested locally on:

• Linux Mint 22.3
• Go 1.26
• Wails 2.12

Verified:

• Group selection
• Exercise generation
• Start practice flow
• Custom text practice
• Session completion
• Session summaries

Screenshots

[AshBuk]

Closing in favour of a focused PR.
Scope: targeted practice only with built-in groups + weak-keys derived from mistake history. No custom groups (and no new file / Go backend storage). No custom-text paste - duplicates the library functionality with minimal added value.
A PR scoped to the above would be very welcome!

Also, this is on me: I should have clearly stated that new features (2k+ lines) are best discussed in an issue before submitting a PR.

Thanks, appritiate your contribution!