feat: add use cases

docs/use_cases.md

@@ -1,4 +1,96 @@
 # Use cases
 
-Use cases are generated from `FRs.md`, and `meta-arch.md` in `gherkin` format.
+Use cases are derived from `FRs.md` (user stories) and `meta-arch.md`. Each Feature name matches the corresponding user story; scenarios focus on observable behavior without restating story details.
+
+```gherkin
+Feature: PDF browser
+
+	Scenario: Open a PDF and navigate pages
+		Given a PDF document is available
+		When the user opens the document
+		Then the first page is displayed
+		And the user can move to the next or previous page
+
+	Scenario: Jump to a specific page
+		Given a multi-page PDF is open
+		When the user selects a specific page number
+		Then that page is displayed
+
+	Scenario: Select a page for signing
+		Given a PDF is open
+		When the user marks the current page for signing
+		Then the page is set as the signature target
+```
+
+```gherkin
+Feature: load signature picture
+
+	Scenario: Import a signature image
+		Given a PDF page is selected for signing
+		When the user chooses a signature image file
+		Then the image is loaded and shown as a signature asset
+
+	Scenario Outline: Handle invalid or unsupported files
+		Given the user selects "<file>"
+		When the app attempts to load the image
+		Then the user is notified of the issue
+		And the image is not added to the document
+
+		Examples:
+			| file            |
+			| corrupted.png   |
+			| signature.bmp   |
+			| empty.jpg       |
+```
+
+```gherkin
+Feature: geometrically adjust signature picture
+
+	Scenario: Resize and move the signature within page bounds
+		Given a signature image is placed on the page
+		When the user drags handles to resize and drags to reposition
+		Then the size and position update in real time
+		And the signature remains within the page area
+
+	Scenario: Lock aspect ratio while resizing
+		Given a signature image is selected
+		When the user enables aspect ratio lock and resizes
+		Then the image scales proportionally
+```
+
+```gherkin
+Feature: graphically adjust signature picture
+
+	Scenario: Remove background
+		Given a signature image is selected
+		When the user enables background removal
+		Then near-white background becomes transparent in the preview
+		And the user can apply the change
+
+	Scenario: Adjust contrast and brightness
+		Given a signature image is selected
+		When the user changes contrast and brightness controls
+		Then the preview updates immediately
+		And the user can apply or reset adjustments
+```
+
+```gherkin
+Feature: draw signature
+
+	Scenario: Draw with mouse or touch and place on page
+		Given an empty signature canvas
+		When the user draws strokes and confirms
+		Then a signature image is created
+		And it is placed on the selected page
+
+	Scenario: Clear and redraw
+		Given a drawn signature exists in the canvas
+		When the user clears the canvas
+		Then the canvas becomes blank
+
+	Scenario: Undo the last stroke
+		Given multiple strokes were drawn
+		When the user chooses undo
+		Then the last stroke is removed
+```