Fix target-grade simulator: make achievability check rounding-consistent

[veniplex]

The dashboard "what do I need?" (Zielnote) simulator reported a target as unreachable in a case where the best achievable final grade rounds to exactly that target.

Problem

The simulator compared the required remaining average against the raw target, but every grade in the app is displayed rounded to one decimal. Example (real user data): 60 ECTS graded at a 2.0 average, 60 ECTS remaining, target 1.5.

• To land on 1.5 you need a 0.99 average on the remaining modules — below the best grade (1.0), so it was flagged "unreachable".
• But the best achievable final is 1.504, which the app displays everywhere as "1,5". So a student who scores straight 1.0s would see a final Schnitt of "1,5" on the dashboard while the goal simulator had said it was impossible — an internal inconsistency.

(The raw math itself was correct: a 1.2 average on the remainder yields a 1.6 final, not 1.5 — 1.2 is actually the requirement for a 1.6 target.)

Change

• Extracted the logic into a tested pure function requiredGradeForGoal in src/lib/grades.ts and round the required grade to one decimal before classifying it, matching the rest of the app.
• The borderline case now reads "you need a 1.0" instead of "unreachable"; safe/unreachable thresholds reuse the existing BEST_GRADE / PASS_THRESHOLD constants.
• src/components/learn/grade-goal.tsx now calls the shared function instead of computing inline.

Verification

• New unit tests in grades.test.ts cover the exact reported scenario (1.5 → needed 1.0; 1.6 → needed 1.2), plus truly-unreachable, safe, and no-remaining-ECTS cases. Full grade suite green (26 tests), tsc --noEmit and eslint clean.

🤖 Generated with Claude Code

https://claude.ai/code/session_01LWLUNyHYuCpouMEUo3P456

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