[docs][stepper] Fix focus management in examples

[silviuaavram]

• I have followed (at least) the PR section of the contributing guide.

This PR addresses focus loss that occurs in interactive Stepper demos when the focused button is removed or disabled after a step transition.

Cases fixed per demo:

• HorizontalLinearStepper — focuses Reset when Finish is clicked (completing all steps); focuses Next when Reset is clicked (returning to step 1); focuses Next when Skip is used and the next step is not optional
• HorizontalNonLinearStepper — refactored helper functions (totalSteps, completedSteps, isLastStep, allStepsCompleted) from functions to derived variables; focuses Reset when all steps are completed; focuses Next when Reset is clicked; focuses Next when navigating back to the first step via Back
• VerticalLinearStepper— focuses Continue when going forward; focuses Reset when Finish is clicked; focuses Continue when Reset is clicked (returning to step 1) or when reaching the first step via Back button; focuses Back when going backward; also removes the Back button from the DOM on the first step instead of just disabling it, so the ref always points to a mounted button
• DotsMobileStepper / ProgressMobileStepper / TextMobileStepper — focuses Next when going back to the first step (Back becomes disabled); focuses Back when reaching the last step (Next becomes disabled)

Also adds aria-labels to the two examples showing a progress indicator:

• ProgressMobileStepper
• DotsMobileStepper

[Copilot]

Pull request overview

This PR updates several Stepper documentation demos to preserve keyboard focus when step transitions disable or remove the currently focused button, by programmatically moving focus to an appropriate remaining control.

Changes:

• Added focus-management refs and useEffect logic to Horizontal/Vertical Stepper demos to move focus to Next/Back/Reset during step transitions.
• Added similar focus-management to MobileStepper demos (dots/progress/text) when reaching the first/last step where one control becomes disabled.

Reviewed changes

Copilot reviewed 12 out of 12 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
docs/data/material/components/steppers/VerticalLinearStepper.tsx	Adds refs + effect to focus Continue/Back/Reset across step changes
docs/data/material/components/steppers/VerticalLinearStepper.js	JS equivalent of the VerticalLinearStepper focus changes
docs/data/material/components/steppers/HorizontalNonLinearStepper.tsx	Adds refs + effects to focus Reset/Next on completion/reset and when navigating back to step 1
docs/data/material/components/steppers/HorizontalNonLinearStepper.js	JS equivalent of the HorizontalNonLinearStepper focus changes
docs/data/material/components/steppers/HorizontalLinearStepper.tsx	Adds refs + effect to focus Reset/Next for finish/reset/skip flows
docs/data/material/components/steppers/HorizontalLinearStepper.js	JS equivalent of the HorizontalLinearStepper focus changes
docs/data/material/components/steppers/DotsMobileStepper.tsx	Adds refs + effect to focus Next/Back when reaching first/last step
docs/data/material/components/steppers/DotsMobileStepper.js	JS equivalent of the DotsMobileStepper focus changes
docs/data/material/components/steppers/ProgressMobileStepper.tsx	Adds refs + effect to focus Next/Back when reaching first/last step
docs/data/material/components/steppers/ProgressMobileStepper.js	JS equivalent of the ProgressMobileStepper focus changes
docs/data/material/components/steppers/TextMobileStepper.tsx	Adds refs + effect to focus Next/Back when reaching first/last step
docs/data/material/components/steppers/TextMobileStepper.js	JS equivalent of the TextMobileStepper focus changes

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Bundle size

Bundle	Parsed size	Gzip size
@mui/material	0B(0.00%)	0B(0.00%)
@mui/lab	0B(0.00%)	0B(0.00%)
@mui/private-theming	0B(0.00%)	0B(0.00%)
@mui/system	0B(0.00%)	0B(0.00%)
@mui/utils	0B(0.00%)	0B(0.00%)

Details of bundle changes

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[Copilot]

Pull request overview

Copilot reviewed 12 out of 12 changed files in this pull request and generated 8 comments.