Enhance logfile detail and add expandable error drill-down in dashboard

[trsdn]

Problem

The current logfile / log view still contains far too little information to diagnose what actually happened.

Important failures and state transitions are too compressed, and when something goes wrong there is not enough context visible in the UI. In particular, it should be possible to inspect detailed error information instead of only seeing a short summary line.

Related background:

• #387 introduced centralized error logging / error viewer
• #471 covers persistence of sprint activity across restarts

This issue is about making the log output itself much more useful and drill-down friendly.

Desired behavior

• Log entries should carry more context and diagnostic detail.
• Errors should be expandable/clickable to reveal the underlying details.
• The log view should help explain why something failed, not just that it failed.

Acceptance criteria

• Log output includes richer structured context for important events (phase changes, issue execution, retries, blocks, merge/pre-merge failures, recovery attempts, decisions).
• Error entries expose detailed drill-down information, e.g. message, cause, relevant command/tool, and stack/details when available.
• The dashboard log view supports expanding or opening a detailed error view from a log row.
• Normal success/info logs remain readable and are not drowned in noise.
• The logging model distinguishes summary rows from detail payloads cleanly.
• Regression coverage is added for the log/event payload shape and the log viewer interaction.