docs: todo.md rewritten around what is left

The four original items are done. What replaces them: the invisible
riser, relative timing, named configurations, frame storage for
forensics, opening exported waveforms, the plot context menu, drag
resize, an event cache with replay, and a preferences dialog.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

todo.md

@@ -1,8 +1,30 @@
-1. better time window management:
-
-- instead of a drop down simple combo should be an advanced sub menu with text input + unit selector (us,ms,s,min) + two button to increase / decrease (factor 10 each time)
-- when `timewindow x sample rate x sample size` is greater than `buffer size` apply downsampling of input signals with low pass filter to match the correct freq: e.g. downsample at 100kSps -> low pass filter at 50kHz
-
-2. improve layout management: instead of a fix number of figure, in the menu add a layout editor option, user can add and remove rows ([x] close to the top left of the row to remove, [+] in the top center and bottom center to add new one [above] and [below])
-3. improve plot font rendering and line rendering
-4. improve plot performances with large data (e.g. at 10MSamples the plot start to be laggy)
+1. fix vertical lines to small to be seen: in some cases (e.g. rect waveform in @mock_upstreamer.py) the vertical lines are to thin to be visible (sometime they glitch in and out of existence):
+   1. the antialisaing strategy with phosphore effect should be applied with more stregth
+   2. the lines should be more tick
+   3. the sample points should be bigger
+2. relative timing:
+   - in order to avoid problem with time jitter user can decide to go in relative time mode
+   - in this mode absolute signal time is only use for export
+   - the plot time depends just on signal sample rate and frame number, e.g.:
+     - user is acquiring a signal with 1MSps and 100_000 samples per udp frame
+     - last frame as number <N> and previous <N-2> in this case a 100_000 indexes must be skipped
+     - time is simply: `((<N_frame> - <N_last>) * <n_samples_per_frame> - <sample_index>) / <sample rate>` (or something like that)
+3. named configurations: in order to have multiples configurations for the same sources configuration can also have comments
+4. optional udp frame storage for debugging of strange behaviours (store last <N>, default = 1000, udp packets binary content on disk for forensic analyss)
+5. open exported binary waveforms to view/analyse, in the source selection dialog there should be the option to open exported waveform (maybe a tab with latest opened file and option to open new ones?)
+6. imporve right click menu on the plots:
+   - use same compact style of main menu
+   - add a [remove signal] entry that open a submenu to delete signasl
+7. in layout edit mode add the option to drag plot border to resize them
+8. add option to automatic save the last <N> triggered acquisitions in a cache folder and let the user replay them
+   - a collapsable pane on the left with the list of the last <N> acqusition with duration number of signals etc
+   - user select one and immidiatly open it
+   - this option would only be available in trigger mode
+9. preferences dialog:
+   - user can choose evnt cache directory (default <home>/.cache/udpscope/<source>/events/)
+   - maximum number of events to cache
+   - disable completly the events storage option
+   - enable/disable relative time mode
+   - interface dark/light/auto color
+   - defaults
+   - other parameters