feat: allocation setting shortcuts for spend and budgeted amount of last month

[morremeyer]

Resolves #44.

[codecov]

Codecov Report

Merging #303 (dd6ce01) into main (ea0c310) will decrease coverage by 1.19%.
The diff coverage is 81.03%.

❗ Current head dd6ce01 differs from pull request most recent head 46588a0. Consider uploading reports for the commit 46588a0 to get more accurate results

@@            Coverage Diff             @@
##             main     #303      +/-   ##
==========================================
- Coverage   98.02%   96.82%   -1.20%     
==========================================
  Files          17       17              
  Lines        1669     1765      +96     
==========================================
+ Hits         1636     1709      +73     
- Misses         26       43      +17     
- Partials        7       13       +6     

Impacted Files	Coverage Δ
pkg/controllers/envelope.go	94.33% <27.27%> (-3.70%)	⬇️
pkg/controllers/budget.go	93.29% <83.09%> (-2.56%)	⬇️
pkg/models/envelope.go	94.44% <90.47%> (-5.56%)	⬇️
pkg/httperrors/errors.go	100.00% <100.00%> (ø)

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