feat: instructors can toggle between their assigned submissions and all submissions

[YheChen]

Proposed Changes

(Describe your changes here. Also describe the motivation for your changes: what problem do they solve, or how do they improve the application or codebase? If this pull request fixes an open issue, use a keyword to link this pull request to the issue.)

Instructors can now switch between all submissions and only the submissions assigned to them in:

• The assignment Submissions table
• The assignment Grades summary table
• The grading view's Next and Previous submission navigation

Instructors continue to see all submissions by default, while TAs retain their existing assigned-submissions default. Admin roles remain excluded because they cannot be assigned as graders.

This PR also:

• Identifies instructor-assigned groupings in submission and summary data
• Preserves saved grading-view submission scope preferences
• Adds regression tests for instructor filtering, navigation, role defaults, and admin exclusion
• Updates the instructor documentation to describe the new controls

This is the second incremental part of #1221.
Ref #1221

Screenshots of your changes (if applicable)

Type of Change

(Write an X or a brief description next to the type or types that best describe your changes.)

Type	Applies?
🚨 Breaking change (fix or feature that would cause existing functionality to change)
✨ New feature (non-breaking change that adds functionality)	X
🐛 Bug fix (non-breaking change that fixes an issue)
🎨 User interface change (change to user interface; provide screenshots)	X
♻️ Refactoring (internal change to codebase, without changing functionality)
🚦 Test update (change that only adds or modifies tests)
📦 Dependency update (change that updates a dependency)
📖 Documentation update (change that updates documentation)	X
🔧 Internal (change that only affects developers or continuous integration)

Checklist

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Before opening your pull request:

• I have performed a self-review of my changes.
  • Check that all changed files included in this pull request are intentional changes.
  • Check that all changes are relevant to the purpose of this pull request, as described above.
• I have added tests for my changes, if applicable.
  • This is required for all bug fixes and new features.
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Questions and Comments

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[coveralls]

Coverage Report for CI Build 30134625234

Coverage decreased (-2.0%) to 88.556%

Details

• Coverage decreased (-2.0%) from the base build.
• Patch coverage: 6 uncovered changes across 1 file (97 of 103 lines covered, 94.17%).
• No coverage regressions found.

Uncovered Changes

File	Changed	Covered	%
app/javascript/Components/Result/result.jsx	24	18	75.0%
Total (7 files)	103	97	94.17%

Coverage Regressions

No coverage regressions found.

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Coverage Stats

Relevant Lines:	52404
Covered Lines:	47758
Line Coverage:	91.13%
Relevant Branches:	2917
Covered Branches:	1232
Branch Coverage:	42.24%
Branches in Coverage %:	Yes
Coverage Strength:	126.85 hits per line

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💛 - Coveralls