feat: add weekly goal setter widget to [SYS_TARGETS] card (#113)

[manjiri27]

Description

Adds a weekly goal-setting widget to the [SYS_TARGETS] card on the user profile page. Users can set a weekly problem-solving target (by difficulty: Total / Easy / Medium / Hard), track their progress via a retro terminal-style progress bar, and reset their goal at any time. Goals persist across sessions via localStorage and auto-reset every Monday.

Linked Issue

Fixes #113

Changes Made

Populated the empty [SYS_TARGETS] card in frontend/user.html with goal setter UI (metric selector, number input, [SET] button)
Added progress display view showing [=====>..........] X% bar, current count, and a celebration message on goal completion
Created frontend/js/user/goal-setter.js to handle all goal logic: save/load/reset via localStorage, weekly auto-reset every Monday, live solved count fetched from /api/user/:username

Type of Change

• Bug fix
• New feature
• UI/Visual update
• Documentation update
• Refactor

Testing

• Tested locally
• Tested on mobile viewport (if applicable)
• No console errors introduced

Checklist

• My code follows the project's coding style
• I have formatted my code locally by running npx prettier --write . before submitting
• I am submitting my PR from a dedicated feature/* branch, not the main branch
• I have performed a self-review of my code
• My changes generate no new warnings or errors
• I have updated documentation if required
• I have linked the relevant issue

Screenshots / Screen Recording

Goal setter (no goal set):

Progress bar (after setting goal):

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