Add the Demand Letter and Fee Structure Certificate screens

[vikrantwiz02]

Summary

• Two new certificate screens, following the same generate -> preview -> download -> history pattern as the existing Bonafide and Paid/Unpaid certificates
• GeneratedCertificatesModal now supports four variants instead of two
• Demand Letter: roll number lookup, a semester picker (re-fetches the fee split and academic-year label on change), and a due-date picker; preview and PDF download are kept visually identical (justified body text, non-wrapping fee table)
• Fee Structure Certificate: roll number lookup and a per-head fee break-up table (Semester-I / Semester-II / Total) for the student's current academic session
• Institute bank account details moved to a standalone "Bank Account Details" panel on the Fee Structure page -- one shared record for every programme and year, not duplicated per session

Test plan

• eslint clean on every touched/new file
• Production build succeeds
• Manually verified preview/PDF parity (alignment, font size, table wrapping) against the reference circular images