fix: resolve RD and FD Calculator buttons not functioning (#3338, #3336)

[ShreyaRHipparagi]

Root cause: Duplicate element IDs in sip.html caused getElementById to attach event handlers to the wrong buttons (Investment Growth/Tax calculators instead of RD/FD calculators).

Changes:

• RD Calculator: Changed button IDs to unique calculate-rd-btn and clear-rd-btn
• FD Calculator: Changed button IDs to unique calculate-fd-btn and clear-fd-btn
• Added missing error message divs (installmentError, rateError, monthsError, principalError, fdRateError, yearsError)
• Updated RDCalculator.js and FDCalculator.js to reference new IDs

Fixes #3338
Fixes #3336

🛠️ Fixes Issue

Fixes: #<issue_number>

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