[abhayrajjais01]

What does this PR do?

• Fixes ui(import): Input custom rescan blockheight as year/month datepicker #638

This pull request implements Part 1 of Issue #638 based on the maintainer-approved plan and the bitcoin.design mockup:

1. Date-to-Blockheight Calculation (src/lib/blockheight.ts):

   • Implements piecewise linear interpolation anchored to key verified Bitcoin milestones (Genesis, Halvings 1–4, SegWit, Taproot).
   • Incorporates a 1-month safety buffer (DEFAULT_SAFETY_BUFFER_MONTHS = 1, as approved by @theborakompanioni) to ensure transactions created early in the selected month are not skipped.
   • For historical dates back to Jan 2009 or prior, cleanly defaults to block 0 (Genesis).
   • Extrapolates future blocks past the latest checkpoint and clamps to currentBlockHeight if available.
   • Includes comprehensive unit tests.

2. Tabbed RescanHeightSelector Component (src/components/import/RescanHeightSelector.tsx):

   • "By creation date" tab:
     • Month and Year dropdown selectors with descending years (current year down to 2009).
     • Informative hint showing the calculated blockheight and confirming the ~1 month safety buffer.
     • "Start at genesis block (Jan 2009)" toggle that locks dropdowns and sets blockheight to 0.
   • "By block number" tab:
     • Retains direct numeric block input with icon addon for advanced users.
     • Uses forceMount with hidden styling to preserve DOM presence and backward compatibility for existing tests and form validation.

3. Form & Review Step Integration:

   • Integrated into ImportDetailsForm.tsx using Controller from react-hook-form.
   • Updated ImportStepConfirm.tsx to display "Block #0 (Genesis)" when blockheight is 0.
   • Added all necessary English translation keys in translation.json.

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Visual Demo

Image Demo:

State	Preview
By creation date (Month / Year selection)
Start at Genesis block toggle
By block number (Direct block input)