Show actionable App errors with expandable diagnostics

[amising6]

Summary

Addresses #369 by replacing raw Databricks App errors with actionable messages while keeping the underlying exception in an expandable Details section.

Changes

Add a shared error helper that separates the user-facing message from technical diagnostics.
Apply it to Unity Catalog browsing, DataflowSpec queries, pipelines, workspace files, SQL warehouses, and unhandled App errors.
Show command and API failures in the response modal with technical output collapsed by default.
Preserve Unity Catalog grant diagnostics alongside the suggested GRANT SQL.
Add regression tests for permission errors, retained diagnostics, and the Details UI.

Validation

Focused Flask tests: 90 passed, 15 subtests passed locally on Python 3.12.
Tested: test_app_error_messages.py, test_app_landing_page.py, test_app_metadata_browse.py, test_app_deploy_form.py, and test_app_onboarding_preview.py.
Python compilation, JavaScript syntax, and git diff --check passed.

[CLAassistant]

All committers have signed the CLA.

[ravi-databricks]

@amising6 this pr cant be merged. Please check #492 verify.

[amising6]

@ravi-databricks I verified #492 at commit 8ea1ed9 locally using Python 3.12 and my Databricks workspace.

The featured demo defaults correctly, catalog access validation works, logs stream, and the Databricks job link is clickable. When the run failed, the App displayed a friendly failure message with expandable technical details, stdout/stderr, and exit code 1. Automated validation also passed: 169 tests and 39 subtests, plus lint and whitespace checks.

The 100-table demo failed at onboard_specs because the workspace rejected the launcher’s client="1" environment:

Workspace doesn't support Client-1 channel for REPL.

[Failed run](https://dbc-dde4f237-1409.cloud.databricks.com/jobs/539729170382885/runs/686357605711888?o=361003152442980)

Successful completion and close-while-running behavior remain unverified. Which supported environment version should I use to finish testing?