fix: move hardcoded config to env vars and replace timeouts with waitForSelector [ GSSOC'26 ]

[rudra3007-pro]

Fixes #519

##Problem
capture.js hardcodes the dev server URL, login credentials, and screenshot output path. setTimeout is used to wait for the sidebar and chat to load, which is fragile on slow machines.

##Changes:
URL, email, password, and output path now read from .env via dotenv with fallbacks
Replaced both setTimeout waits with waitForSelector so the script reacts to actual DOM readiness
Wrapped browser.close() in a finally block so the browser always closes even if the script throws
Auto-creates the output directory if it doesn't exist
Throws a clear error if login appears to fail (sidebar never appears after submit)

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• backend/capture.js

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[rudra3007-pro]

@UTKARSHH20 Please review this PR

[rudra3007-pro]

@UTKARSHH20 Please review this PR

[rudra3007-pro]

@UTKARSHH20 Please review this PR