fix: improve dark mode consistency and resolve UI visibility issues

[faizaanjum7]

Description

Intially, i was working on implementing dark mode support across the project. after completing the development, i found that a duplicate dark mode implementation PR had already been merged. instead of completing discarding the work, i adapted the changes to focus on improving and polishing the current dark mode experience by fixing bugs, inconsistent styling, and accessibility/visibility issues

The goal of this PR is to make dark mode visually consistent, readable, and responsive across all major pages and components

Related Issue

Closes #6

Changes Made

• Improved dark mode consistency across Dashboard, Tasks, Routine Builder, Navbar, Login, and mobile sidebar
• Fixed low-contrast cards and task containers that appeared washed out in dark mode
• Refined Weekly Schedule grid styling
• Improved visibility of inputs, placeholders, labels, and buttons in dark mode
• Fixed duplicate/overlapping theme toggle behavior in mobile navbar
• Improved hover states and dark-mode border visibility
• Fixed several dark mode inconsistencies introduced during merge conflicts/rebase

Screenshots

before

after

✅ Checklist

• Code runs locally
• Followed project structure
• No console errors
• Properly tested changes
• Linked the issue

[faizaanjum7]

Hi @aryandas2911
i have resolved the merge conflicts, please review and merge, thank you