Sequence alignment: pixel-perfect rendering, consensus propagation, and direct FASTA upload

Several improvements to sequence alignment support:

• Pixel-perfect cell rendering — the sequence alignment viewer no longer shows thin phantom lines between residue cells at any zoom level. Each residue is now encoded as a crisp integer-aligned rectangle in the SVG, eliminating sub-pixel antialiasing artefacts.

• Consensus propagation — alignment properties can now be propagated to internal nodes using majority-vote consensus directly from the tree detail page. Click Propagate on any sequence property and choose Sequence → Majority-vote consensus. The seqtype (amino acid vs nucleotide) is detected automatically from the stored sequences.

• Direct FASTA upload from the tree page — uploading a FASTA alignment from the tree detail panel now opens a configuration step where you can choose the sequence type, output property name, and whether to compute consensus for internal nodes, before submitting the annotation job.