[WIP] Refactor subject management and add ICS export features

[konnn04]

Description

Modularize the GPA improvement suggestion system with manual links between old and new subjects. Add reminders for ICS export and fix the issue with "Exempt" subjects.

New features:

• Add manual links for improved subjects using improvesSubjectId and a UUID for each subject.
• Add new components: GPALadder, ImprovementDetailsDialog, SubjectDialog, LinkImprovementDialog, ProgressTimeline, and UserIdentityDialog.
• Improve ICS export by adding VALARM reminders, following RFC 5545 line folding, and allowing a custom calendar name.

Bug fixes:

• Ignore "Exempt" subjects (character M) when calculating GPA.
• Use effectiveStudentId instead of studentId in saveData to support account switching.
• Round the GPA before checking the academic ranking table.

Type of change

• Bug fix
• New feature
• Breaking change
• Performance improvement
• Documentation

Checklist

• Tested on Chrome (pnpm dev)
• Tested on Firefox (pnpm dev:firefox)
• pnpm ultracite:check passed
• pnpm compile passed
• Commits follow the Conventional Commits format

Ref

#57 #56