refactor(domain): unify ETR calculation — both UIs now use calc_etr_str

Before:
  - PyQt: inline ETR using chars-based formula
  - WebUI: Job.estimated_time_remaining using progress-based formula
  (different formulas → different ETR estimates)

After:
  - Both UIs call domain.progress.calc_etr_str(elapsed, done, total)
  - Same formula, same ETR, single source of truth
  - WebUI now stores etr_str on Job and displays it directly
  - Job.estimated_time_remaining property kept for backward compat

domain/progress.py: ProgressTracker class + calc_etr_str function
1053 tests pass.

tests/test_domain_progress.py

@@ -0,0 +1,71 @@
+from __future__ import annotations
+
+import time
+from unittest.mock import patch
+
+from abogen.domain.progress import ProgressTracker, calc_etr_str
+
+
+class TestCalcEtrStr:
+    def test_returns_processing_when_not_started(self):
+        assert calc_etr_str(0.0, 0, 100) == "Processing..."
+
+    def test_returns_processing_when_elapsed_too_short(self):
+        assert calc_etr_str(0.3, 50, 100) == "Processing..."
+
+    def test_returns_processing_when_done_zero(self):
+        assert calc_etr_str(2.0, 0, 100) == "Processing..."
+
+    def test_returns_zero_when_complete(self):
+        assert calc_etr_str(10.0, 100, 100) == "00:00:00"
+
+    def test_returns_zero_when_overcomplete(self):
+        assert calc_etr_str(10.0, 150, 100) == "00:00:00"
+
+    def test_half_done(self):
+        # 10s for 50 chars -> 10s remaining -> 00:00:10
+        assert calc_etr_str(10.0, 50, 100) == "00:00:10"
+
+    def test_one_third_done_large(self):
+        # 100s for 1000 chars out of 3000 -> 200s remaining
+        assert calc_etr_str(100.0, 1000, 3000) == "00:03:20"
+
+    def test_hours_format(self):
+        # 3600s for 1000 chars out of 4000 -> 3 * 3600 = 10800s remaining
+        assert calc_etr_str(3600.0, 1000, 4000) == "03:00:00"
+
+    def test_minutes_and_seconds(self):
+        # 60s for 100 chars out of 200 -> 60s remaining
+        assert calc_etr_str(60.0, 100, 200) == "00:01:00"
+
+
+class TestProgressTracker:
+    def test_percent_at_zero(self):
+        t = ProgressTracker(total_chars=1000)
+        assert t.percent == 0
+
+    def test_percent_half(self):
+        t = ProgressTracker(total_chars=1000)
+        t.update(500)
+        assert t.percent == 50
+
+    def test_percent_capped_at_99(self):
+        t = ProgressTracker(total_chars=100)
+        t.update(100)
+        assert t.percent == 99  # matches original behavior
+
+    def test_etr_processing_at_start(self):
+        t = ProgressTracker(total_chars=1000)
+        t.update(0)
+        assert t.etr_str == "Processing..."
+
+    def test_etr_computes_correctly(self):
+        t = ProgressTracker(total_chars=200)
+        with patch("abogen.domain.progress.time") as mock_time:
+            mock_time.time.return_value = t._start_time + 2.0
+            t.update(100)
+            assert t.etr_str == "00:00:02"
+
+    def test_zero_total_chars(self):
+        t = ProgressTracker(total_chars=0)
+        assert t.percent == 0