Add text and HTML representations to DE model classes

[Zethson]

Closes #611

Inspecting a differential expression model now shows a summary of the input data, the design and the fit state, both as plain text and as an HTML table in Jupyter.

>>> edgr
EdgeR
    Data          1,525 obs × 27,085 vars
    Layer         X
    Design        1 + Efficacy + Treatment
    Variables     Efficacy, Treatment
    Coefficients  Intercept, Efficacy[T.PD], Efficacy[T.PR], Efficacy[T.SD], Treatment[T.Chemo]
    Fitted        yes

Both representations are driven by a single _summary() hook, so MethodBase covers the simple tests and LinearModelBase adds the design rows.
The HTML uses inline styles relative to the surrounding text, so it renders on light and dark themes and is not affected by the notebook's own table CSS.

LinearModelBase gains a public is_fitted property to back the Fitted row; PyDESeq2.plot_disp_ests now uses it instead of probing for self.dds.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 96.15385% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 79.46%. Comparing base (40dfeb6) to head (1ca8060).
⚠️ Report is 10 commits behind head on main.

Files with missing lines	Patch %	Lines
...rtpy/tools/_differential_gene_expression/_edger.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1070      +/-   ##
==========================================
+ Coverage   74.15%   79.46%   +5.30%     
==========================================
  Files          52       53       +1     
  Lines        7291     7545     +254     
==========================================
+ Hits         5407     5996     +589     
+ Misses       1884     1549     -335     

Files with missing lines	Coverage Δ
...ertpy/tools/_differential_gene_expression/_base.py	95.93% <100.00%> (+5.23%)	⬆️
...y/tools/_differential_gene_expression/_pydeseq2.py	91.83% <100.00%> (+0.17%)	⬆️
...ools/_differential_gene_expression/_statsmodels.py	100.00% <100.00%> (ø)
...rtpy/tools/_differential_gene_expression/_edger.py	16.92% <0.00%> (-0.27%)	⬇️

... and 8 files with indirect coverage changes

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