fix: strict input validation on POST /api/goals

[advikdivekar]

Problem

POST /api/goals accepted arbitrary values with no bounds checking:

• target: 0 → division by zero in the frontend progress bar (0 / 0 * 100 = Infinity%)
• target: -1 → a goal with current: 0 displayed as 100% complete
• target: 0.5, NaN, 999999999 → silently stored
• title had no length cap — a multi-MB string would be persisted and break layout
• A malformed JSON body produced an unhandled 500 instead of a 400

Fix

src/app/api/goals/route.ts

• req.json() wrapped in try/catch → returns 400 Invalid JSON on parse failure
• title: must be a non-empty string, trimmed, ≤ 100 characters
• target: must be an integer in [1, 10_000] — rejects 0, negatives, floats, NaN, and overflow values
• unit: clamped to 30 characters silently
• recurrence: unknown values default to "none" explicitly
• All bounds defined as named constants at the top of the file

Test plan

• POST /api/goals with target: 0 returns 400
• POST /api/goals with target: -5 returns 400
• POST /api/goals with target: 0.5 returns 400
• POST /api/goals with target: 999999999 returns 400
• POST /api/goals with a 101-character title returns 400
• POST /api/goals with a whitespace-only title returns 400
• POST /api/goals with malformed JSON returns 400
• Valid goal creation still works as expected

Closes #454

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[Priyanshu-byte-coder]

Merge conflict — rebase on main to resolve. The logic and implementation are correct; once conflict is cleared this can merge immediately.

[advikdivekar]

@Priyanshu-byte-coder resolved the merge conflicts