Upgrade date pickers dayjs resizable panels

[camd]

Only the last commit applies to this. This PR sits atop #9114

Summary

This PR upgrades several frontend packages to modernize the codebase:

Package Changes

• Added: @mui/x-date-pickers (8.21.0) - MUI date picker components
• Added: dayjs (1.11.19) - Lightweight date library
• Added: react-resizable-panels (3.0.6) - Modern resizable panel library
• Removed: moment - Replaced by dayjs (smaller bundle size)
• Removed: react-dates - Replaced by MUI date pickers
• Removed: react-split-pane - Replaced by react-resizable-panels

Code Changes

• Migrated date handling from moment to dayjs across the codebase
• Created ui/helpers/dayjs.js helper module for consistent dayjs configuration
• Replaced react-dates DateRangePicker with MUI @mui/x-date-pickers in intermittent-failures
• Replaced react-split-pane with react-resizable-panels in the main job view App.jsx
• Updated CSS for new resizable panels styling
• Added Jest mock for react-resizable-panels

Files Modified

• 18 files changed across helpers, components, and configuration

[codecov-commenter]

Codecov Report

:x: Patch coverage is 64.06250% with 23 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 81.43%. Comparing base (a82c683) to head (96dcbaf).

Files with missing lines	Patch %	Lines
ui/intermittent-failures/DateRangePicker.jsx	0.00%	10 Missing :warning:
ui/job-view/App.jsx	70.58%	5 Missing :warning:
ui/intermittent-failures/helpers.jsx	25.00%	3 Missing :warning:
ui/intermittent-failures/DateOptions.jsx	0.00%	2 Missing :warning:
ui/helpers/taskcluster.js	50.00%	1 Missing :warning:
ui/login-callback/LoginCallback.jsx	66.66%	1 Missing :warning:
ui/perfherder/graphs/GraphsContainer.jsx	75.00%	1 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9115      +/-   ##
==========================================
+ Coverage   80.01%   81.43%   +1.42%     
==========================================
  Files         601      602       +1     
  Lines       32715    33758    +1043     
  Branches     3314     3007     -307     
==========================================
+ Hits        26178    27492    +1314     
+ Misses       6371     6112     -259     
+ Partials      166      154      -12     

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