Fix GPSTimeStamp for fractional seconds stored as low-denominator rationals

[tonimelisma]

The convertToTimestampString converter in helpers.go used ratNum() (numerator
only) for all three components, then formatted with %02d. It had a follow-up hack
that detected 10-character results and inserted a decimal point, e.g.:

4279/100 → numerator 4279 → "13:03:4279" (10 chars) → "13:03:42.79"  ✓

This only works when the seconds numerator happens to be exactly 4 digits. A rational
like 181/5 (= 36.2 seconds) has a 3-digit numerator and falls through:

181/5 → numerator 181 → "22:22:181" (9 chars) → no decimal inserted  ✗

Fix: compute the seconds value properly using toFloat64() (already used elsewhere
for GPS coordinate conversion) and format with strconv.FormatFloat(secs, 'f', -1, 64).
Integer seconds (e.g. 47/1) still produce "47" with no trailing decimal; fractional
seconds (e.g. 181/5) produce "36.2".

The existing goexif/has-lens-info.jpg golden test (seconds = 4279/100 = 42.79)
continues to pass as before.

This bug was discovered while adding HEIF support and testing against an iPhone 15 Pro
photo whose GPS log records seconds as 181/5.

[bep]

This needs to fix a failing test case.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.40%. Comparing base (93d0b00) to head (13c518b).
⚠️ Report is 11 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #56      +/-   ##
==========================================
+ Coverage   79.38%   79.40%   +0.01%     
==========================================
  Files          14       14              
  Lines        1606     1607       +1     
==========================================
+ Hits         1275     1276       +1     
  Misses        228      228              
  Partials      103      103              

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[bep]

I'll handle this in #58