Add explicit initial grain_mass/water_mass to the Sud doc

Sud now parses top-level grain_mass/water_mass (sibling to pot_mass/
pot_material/L/Td, defaulting to 0/0) - what's actually in the pot before
the brew starts, as opposed to default.step's grain_mass/water_mass, which
is just inert template filler. All sude/*.json docs gain explicit values
matching what their first step already resolved to.

tasks/sud.py's apply_plant_params() now takes grain_mass/water_mass
directly instead of a step dict: on_step_changed() still passes the active
step's own (these vary as malt goes in/water boils off), but the Load
handler now reads Sud.grain_mass/water_mass directly instead of parsing
schedule[0]. The GUI's status-line mass preview (shown immediately on
Load, before Start) does the same.

Also fixes a latent bug found along the way: _continue_forecast_after_confirm()'s
reconstructed sub-doc was missing L/Td/grain_mass/water_mass entirely,
silently falling back to generic defaults instead of the real Sud's own
values - invisible only because every current sude/*.json happens to use
those same defaults.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DkkuG48uHFCGKe6dPSERFk

sude/sud_0010_downloaded.json

@@ -5,6 +5,8 @@
 	"pot_material": "Edelstahl 18/10",
 	"L": 0.2,
 	"Td": 30,
+	"grain_mass": 0,
+	"water_mass": 22,
 	"default": {
 		"step": {
 			"descr": "Put description here",